The science of colour mixing : a manual intended for the use of dyers, calico printers and colour chemists

Front matter

= o) ZA oa | S) M fa sa H

Fronttsprece. Plate I.

: i

Preface

In introducing a new book on Colour Science at the present time, one almost requires to do so with a preface of apology, as there are already many excellent works on the subject. But the writer, along with other colourists, has experienced the want of a book dealing in a practical manner with the science of mixing ordinary colours and dyes, and thus made applicable to the common everyday duties of the dyer and the colourist. Theoretical knowledge alone cannot make a successful colour mixer ; but it certainly proves of great value in explaining the true causes of failure, and in directing the conditions which lead to SUCCESS.

The writer here wishes to express his thanks to the firm of Meister Lucius & Briining, who have very kindly furnished him with the beautiful dyed patterns to be found in the Appendix.

It is hoped that the treatment of the subject in these pages may prove helpful, not only to the practical colourist, but also to students of our Textile Colleges, by forming a useful complement to their class lectures.

Denk: LEA BANK,

Rosstyn, MIDLOTHIAN, May, 1900.

Chapter I

Colour a Sensation—Light Waves—Objects Luminous and Illuminated—Colours of Illuminated Bodies—Production of Colour by Absorption—Diffraction—Dispersion—Fluorescence—Colours of Opaque and Transparent Bodies—Surface Colour . pages 1-10

Chapter II

Analysis of Light — Spectrum — Homogeneous Colours — Ready Method of obtaining a Spectrum—Simple Absorption Spectra —Aid of Spectroscope : , f é : . pages 11-18

CHAPTER ITT.

Examination of Solar Spectrum— Dark Lines— Locality of the Colours— The Spectroscope: Its Construction — Absorption Spectra—Colourists’ Use of the Spectroscope. . pages 19-29

Chapter IV

CoLouR By ABSORPTION.—Absorption of Colour produced by Admixture—Absorption Spectra—Solutions and Dyed Fabrics— Luminosity Curves—Absorption Curves—Dichroism—Dichroie Coloured Fabrics in Gaslight . é : : - pages 30-56

CHAPTER:

Colour Primaries of the Scientist versus the Dyer and Artist—Colour Mixing by Rotation and by Dyeing—Secondary and Tertiary Colours—Constants—Hue, Purity, Brightness—Tints, Shades, Seales, Tones, Sad and Sombre Colours — Complementary Colours . ‘ 5 : ; , , : : . pages 57-81

Chapter VI

CoLour Mixine.—Mixing Qualities of Colours—Pure and Impure Green, Orange and Violets—Large Variety of Shades from Few Colours—Consideration of the Practical Primaries, Red, Yellow and Blue : 5 : : ; : . pages 82-100 b

CONTENTS.

Chapter VII

Secondary Colours — Orange — Green — Violet — Nomenclature of Violet and Purple Group—Violet from Rhodamine Pink and Wool Green—Purple—Tints and Shades of Violet—Changes in Artificial Light . : : , ‘ : . pages 101-109

CEHAPT BE: Vn.

TERTIARY SHADES.—Broken Hues—Maroons—Browns—Citrines and Olives—Absorption Spectra of Tertiary Shades—Dyed Patterns pages 110-121

Appendix

IV. Plates with Dyed Specimens illustrating text.

LIST OF ILLUSTRATIONS.

. Refraction of light by prism ,

. Showing homogeneous nature of Asie um Baloute

. Simple method of obtaining a spectrum -

. Refraction of light

. Refracting angle of prism .

. Simple absorption spectra—white, bite, oe

. Solar spectrum showing principal dark lines and the teeantes

of the fundamental colours .

. One prism spectroscope

. Two prism spectroscope

. Direct vision spectroscope

. Absorption spectra of picric acid aed macau aise ane

strong . ;

. Chromatic circles showing formation of green

. Absorption spectra showing formation of green

Total absorption by combining red and green

. Absorption spectra showing formation of a comport Bins . Chromatic circles showing formation of indigo blue

8. Spectrum of ruby glass light 9. Spectrum of dilute magenta light

. Absorption spectrum of magenta

. Absorption spectrum of green .

. Absorption spectra of dyestuffs, ze. eosine, azo voMown naphthol yellow, malachite green, Victoria blue, methyl violet ‘ :

. Absorption spectra of ehicornie ehleE ae :

. Absorption spectra of rhodamine

. Lambert’s method of mixing colours

. Diagram illustrating Lambert’s method :

. Diagrams showing formation of secondary and eguearey, colours

. Simple chromatic Sree

LIST OF ILLUSTRATIONS.

. Chromatic circle showing complementary shades

. Absorption spectrum of compound red

. Chromatic circles of magenta and picric acid

. Chromatic circles showing formation of a composite blue

. Absorption spectra of malachite green, methyl violet and composite blue F ‘ -

. Absorption spectra showing EORAIaHiOn of orange ad scone

. Absorption spectra of composite violet

. Absorption spectrum of a maroon

. Absorption spectrum of a russet

. Absorption spectrum of a russet composed oo orange G. ail Victoria blue .

. Absorption spectrum of alee

. Absorption spectra of two shades of Bee green, of Einone composition . é : : : - °

COLOURED PLATES.

PLATE I. Frontispiece: Primary, secondary and tertiary colours (Brewster theory).

II. Six figures illustrating dichroism i in colours. III. Different results obtained in mixing coloured lights, compared with dyes.

IV. Showing tertiary shades, russet, citrine, slate. V. Illustrating suitable and unsuitable colour-mixing.

DYED SPECIMEN PLATES.

PLATE J. (4) Victoria rubine. (2) Acid green. (3) Black. (4) Light sky blue. (5) Citrine. (6) Russet. (7), (8), (9) Soft greys, light, mid and dark. . (10) Orange brown. (11) Olive sage.

THE

Chapter I

COLOUR A SENSATION—LIGHT WAVES—OBJECTS LUMINOUS AND ILLUMINATED—COLOURS OF ILLUMINATED BODIES—PRO- DUCTION OF COLOUR BY ABSORPTION—DIFFRACTION— DISPERSION—FLUORESCENCE—COLOURS OF OPAQUE AND TRANSPARENT BODIES—SURFACE COLOUR.

BEFORE proceeding at once to study the principles which underlie the art of mixing colours and dyes, it may be well to review a few elementary facts in regard to colour itself.

§ 1. COLOUR A SENSATION.—The science of colour teaches us that what we call colour is merely a sensation, and has no actual existence in the external world. Colour does not form an inherent physical quality of the coloured body. The sensation of colour is produced in a variety of ways, but pre-eminently by the action of Light on the retina of the eye.

Luminous bodies transmit to the light-bearing medium (luminiferous ether) a state of excessively rapid vibration, and when these vibrations or ‘‘ waves’ impinge upon the fibres of the optic nerve they produce an excitement of that nerve, which, on being conveyed to the brain, is in some mysterious manner translated into the sensation we call laght and colowr. Colour is, however, independent of light, and may be produced by external means. ‘To “see stars,” when the eye receives a blow, is a common enough expression, describing a well-known phenomenon.

If the eyeball be gently pressed with the fingers various different colours are seen, which, as Newton himself described, ‘“‘ resemble the colours of a peacock’s tail”. As early as 1795, Pfaff, a German physicist, showed that by means of an electric shock on the eyes a series of flashes of light were visible in a totally dark room. The sensation of colour is produced even without external aid, as when the sick person sees certain undefined coloured images floating before the eyes; a fact which, not unusually, gives rise to the idea of seeing ghosts. These few examples go to prove that what we call light and colour are merely sensations within the eye itself, caused by the optic nerve being excited, and have really no external existence.

§ 2. The Undulatory or Wave Theory of Light beautifully explains the many complex phenomena of light, but the subject of colour physics lies beyond the scope of this little manual. The light waves are of different lengths, varying from ss3¢y0 Of an inch to about gsdoo Of an inch. The longest undulations which affect the nerves within the retina produce the sensation of red, while the shortest produce the sensation of violet. These light waves may also be indicated by their speed, or rapidity of vibration ; the waves producing the sensation of red being the slowest, while the most rapid undulations give rise to the sensation of violet. Vibrations which are slower, and therefore longer, than those of the red waves do not affect the eye as light, but are invisible rays of heat ; while waves that are shorter and more rapid than even the extreme violet are also invisible, and are termed the chemical or actinic rays When we speak of slow waves of light we must remember that the term is purely a relative one, as the slowest visible light waves producing the sensation

LIGHT WAVE THEORY. 3 of red undulate at the immense speed of 390,000,000,000,000 times per second of time. Or in other words, when the sensitive retina of the eye receives 390 billions of beats or vibrations per second, there is produced the sensation of a red colour. If the vibrations increase in rapidity to 770,000,000,000,000 per second the eye perceives a violet colour. Between these two extremes of wave speeds there lies an indefinite variety of waves of different velocities or wave-lengths, all merging into each other, and producing the sensations of orange, yellow, green, blue and violet colours. The shorter the wave-length of the colour the further will it be removed from red. § 3. OBJECTS LUMINOUS AND ILLUMINATED. —Every object we see is visible either because it gives out light of itself and is therefore self-luminous, or because it reflects light from its surface. The flame of a candle or a lamp, a glowing piece of coal, or the sun, are simple examples of self-luminous objects which emit streams of luminous rays in all directions. Colours are often described as luminous when they reflect a large amount of light to the eye, and are clear and bright.

Illuminated bodies are those which reflect borrowed light to the eye, and the quantity and quality of the reflected light vary with each object. A piece of polished metal, or a sheet of glass, for example, when held at the proper angle reflects nearly all its incident light; while, on the other hand, a piece of black velvet reflects no light whatever, and gives to the eye the impression of absolute darkness. The less light the object reflects the less visible it becomes. This may be illustrated in a simple manner by allowing a drop of oil to fall on a piece of white paper. By doing so the innumerable little surface reflections of the paper are destroyed, and the surface therefore can no longer reflect to the eye its incident light. Instead of being reflected, the light is transmitted through the oiled part, and is lost to the eye which views the surface. This explains the dark grey appearance of an oiled spot on paper. The paper is thereby rendered less opaque, and this simple fact forms the basis of the oiled tracing paper or cloth employed by architects and engineers. An experlment the reverse of this may be made with a piece of colourless transparent glass. The light is freely transmitted through the glass ; and, if no ray of light be reflected from its surface, the glass itself will be invisible. If, however, the glass be crushed to a fine powder, so that innumerable little particles of it reflect the light, then it will present the appearance of a white and almost opaque body.

§ 4. COLOURS OF ILLUMINATED BODIES.— When daylight is passed through a glass of perfectly pure water, it will appear absolutely clear and colourless; but if the light be transmitted through a thickness of three or four feet of the same pure water, it will be observed that the light is no longer colourless, but has acquired a decidedly bluish tinge. Some of the rays of light, in passing through the water, have become absorbed or quenched, leaving the bluish-coloured rays to predominate. If, to the glass of pure water, we add a few drops of magenta solution, we will observe the same phenomenon of absorption, but intensified to a much greater degree. The white light, in passing through the magenta solution, has become robbed of its bright yellow, yellow-green and green rays, and the remaining red, blue and violet-coloured rays combining together in the eye give the sensation of the bluish red or magenta colour. The different quality of the light reflected or transmitted by bodies gives to them their variety of colour. If a substance reflects the rays of the sun all in equal proportion, it will then appear to the eye of the same colour as the light which illuminates it, ie. white or grey. Thus, objects which are white by daylight appear yellow by yellow light, green by green light, or

COLOURS OF ILLUMINATED BODIES. 5 red by red light. But, if the body reflects, as in the majority of cases, some rays in larger proportion than others, it will appear coloured, and that colour will consist of the mixture of all the coloured rays which are reflected. We have already seen (§ 2) that white sunlight consists of six principal colours—red, orange, yellow, green, blue and violet —all marvellously blended together, and, by the admixture of these colours in various proportions, is produced all the endless variety and beauty of colour to be found in art and nature. But if the light under which a coloured material is examined be composed solely of one kind of coloured rays, or what is termed a monochromatic light, then arises a very different state of matters. The material, no matter how brightly coloured or dyed it may be in daylight, will reflect only that single colour, or none at all. Red, yellow and blue colours, when examined under a ruby lamp, will be found to lose their bright variety of hue. The red will appear a pale red, the yellow appears a lighter tint of red, while the blue appears a black. A simple and instructive experiment is to take a pattern sheet containing a variety of beautiful dyed colours and view it under a monochromatic yellow light, obtained by sprinkling common salt on the wick of a spirit lamp. It will be seen that, under such a light, colours cannot be distinguished from each other ; reds, blues, pinks, greens, etc., all appear of the same yellowish grey, deepening into black. Such an experiment shows to us that variety of colour depends altogether upon the quality of the light which illumines, and that the wide variety of hues seen in daylight is owing to the great number of different kinds of coloured rays, of which the light itself is composed. The slight differences noticed in the appearances of dyed shades, especially the blue and violet class, when viewed under artificial lights are due to the same cause; i.e., a deficiency in the light of certain blue and violet coloured rays in comparison with sunlight. The endless number of hues present in ~ nature arise from the different degrees and methods in which the light is affected.

§ 5. PRODUCTION OF COLOUR.—The colours of natural objects are produced, not by one method alone, but by several different phenomena of light. These, with the exception of absorption, which is of special interest to dyers and all colourists, will be but briefly noticed here; and, should the student desire further information on these beautiful and interesting phenomena, he is referred to the larger text-books on Colour.

§ 6. Absorption.—This is the commonest way in which colour is produced. The colours of dyes, pigments, stained glass, the innumerable tints of flowers, leaves, rocks, etc., are all examples of the selective absorption of certain coloured rays of the sunlight, with the reflection or transmission of others. For example, the blue pigment known as French ultramarine reflects to the eye nearly 40 per cent. of the green-blue, blue and blue-violet rays; while the remaining rays, consisting of red, orange and yellow, are absorbed within the pigment. Thus the blue colour of ultramarine is due to its quenching all those rays from red to yellow, and reflecting the remainder, i.e., from green to violet, which combine, within the eye, to give the sensation of an ultramarine blue colour.

Another simple example may be seen in the brilliant pigment, emerald green. This colour is found to reflect to the eye nearly 70 per cent. of the pure yellow-green, green and blue-green rays of the spectrum, and 25 per cent. of the blue and violet; while the red, orange red and orange are completely extinguished.

Likewise the brilliant red colour known as vermilion absorbs all the violet, blue, green-blue and green rays, leaving only the yellow, orange and red rays to be reflected. The

PRODUCTION OF COLOUR. i hight reflected from vermilion consists of 90 per cent. of the orange and red rays, hence its characteristic scarlet-red colour. As this phenomenon of absorption of certain rays, with the reflection of the others, produces the innumerable shades commonly found in nature, and gives to the artist and the dyer their many varieties of colour, it is by far the most important of all colour phenomena to the colour mixer, and will accordingly be treated specially in a subsequent chapter.

§ 7. Diffraction, or Interference. —'This phenomenon, which depends upon the behaviour, or what is termed the interference, of the waves of light, under special circumstances, gives rise to the most beautiful display of colours. The gorgeous hues of the soap bubble, or tar films upon water, the feathers of the peacock, humming bird, and other foreign birds, the iridescent colours on the wings of beetles, of antique glass, mother-of-pearl, the delicate tints of the opal, etc., are all examples of “interference” or diffraction colours, The beautiful colour combinations seen by viewing crystals in polarised light are examples of the same class.

§ 8. Dispersion.—As this third phenomenon of light displays the most exquisite colours, it forms, like diffraction, one of the most fascinating studies for the scientific colourist. In this method of colour production the rays of sunlight undergo neither absorption nor interference, but are simply separated, or dispersed, on account of their different wave-lengths, or what is termed refrangibility. The beautiful band of colours known as the spectrum, obtained by passing light through a prism (see Chap. II.), the colours of the rainbow, or the sparkling dewdrop, glowing with emerald and ruby, are examples of dispersion colours.

§ 9. Fluorescence. — Certain substances and liquids, when illuminated with daylight, emit a peculiarly coloured light, which is termed fluorescence. A piece of uranium glass and a solution of sulphate of quinine give respectively a bluish-green and a purplish-blue fluorescent light. Crystals of anthracene show a beautiful violet fluorescence, while some of the coal tar colours, such as the eosines, rhodamine, etc., “‘fluoresce” an orange or reddish hue. The bluish ‘‘ bloom”’ seen on mineral oils is also due to the same phenomenon. Resorcine blue and uranin yellow—both aniline colours—are splendid examples of fluorescent substances. An alcoholic solution of chlorophyll, or the green of leaves and plants, shows a claret-red fluorescence. This phenomenon is closely related to that of phosphorescence.

§ 10. COLOURS OF OPAQUE BODIES.—An opaque body which reflects to the eye from its irregular surface all the rays of incident light in equal proportion, will appear of the same colour as the light which illumines it. Thus a piece of chalk or white paper is white in daylight, yellowish in a yellow artificial light, red in a red light, or green in a greencoloured light. But, in most cases, objects absorb within themselves certain of the rays of light, and reflect the remainder, which combine together, as they enter the eye, to produce what we know as the colour of the object. Vermilion and aniline scarlet, for example, quench or absorb all the coloured rays of the spectrum, except those of the red and orange red, which are reflected, and these rays give to them their characteristic red colour. If, however, a piece of cloth dyed scarlet, or an object painted with vermilion, be illumined with a green or a blue light, they fail to reflect any of these blue or green coloured rays, and consequently they appear black. This partly explains the reason why red objects, such as a stick of red sealing-wax, a scarlet poppy, or a soldier’s red tunic appear black when seen late in the evening when the sun has set, or in the dusk when the daylight is of a very blue quality.

Objects which absorb to themselves all the rays of light

SURFACE COLOUR. 9 and reflect none to the eye are therefore devoid of all light or colour and appear black ; and the state where an equal proportion of rays is absorbed, and the other reflected is termed grey ; being intermediate between white and black.

§ 11. BODY COLOUR.—TIf the light penetrates for a short distance into the substance, and is then reflected out again, it will generally appear coloured if any selective absorption takes place within the body of the substance. That is, if any of the light rays are absorbed during their passage, the substance will appear coloured by the light which is reflected.

§ 12. Colour of Transparent Bodies.—If{ a transparent substance absorbs certain coloured rays, and transmits others, it will appear coloured from the combination of the transmitted rays.

§ 13. Surface Colour.—Other substances reflect certain rays from their surfaces and transmit the remaining rays. Thus, for example, gold reflects from its surface the orange and yellow rays which give to it the characteristic golden colour ; but, if gold be made sufficiently thin—as in gold leaf —to become transparent, it will be found to transmit the blue and greenrays. This phenomenon of swrface colour, or what is sometimes termed metallic reflection, may be observed in many of the coal tar colours, such as magenta crystals, eosine, methyl violet, some aniline greens, blues, and others. Colours of such a nature appear of a different hue according as they are viewed by transmitted or reflected light. Indigo powder, for example, is a blue colour, yet when it is pressed so as to show a polished surface, or when it is crystallised, it gives a coppery reflection. The crystals of magenta show a dark green reflex like the wings of certain beetles ; while they transmit when dissolved a beautiful magenta-red colour. Methyl violet reflects a clear, greenish-yellow light. Many aniline greens and blues, such as malachite or benzaldehyde green, victoria green, china green, aniline blue, alkali blue, etc., show a coppery red or a brownish metallic lustre. Although dyers and colourists soon get accustomed to these phenomena, it is nevertheless remarkable that a green crystal should give a red solution, and a greenish-yellow substance give a beautiful violet. These facts seldom fail to arouse astonishment among those who see the aniline dyes dissolved for the first time. This reflex depends greatly upon the chemical constitution of the colour salt; thus, with the malachite green group the crystallised oxalate salts show a golden yellow, a yellow green, or a green lustre, while the zinc double salts of the same bases show generally a coppery or a red-brown reflex. Metallic reflection colours may be readily seen by making a strong alcoholic solution of these dyes, and allowing a few drops to evaporate on a glass plate. Their reflected and transmitted colours can thus be readily examined. Generally, the colour of the light transmitted and of the light reflected are complementary to each other, 7.e., if the two coloured lights were combined they would again form white light.

ANALYSIS OF LIGHT—SPECTRUM—HOMOGENEOUS COLOURS— READY METHOD OF OBTAINING A SPECTRUM—SIMPLE ABSORPTION SPECTRA—AID OF SPECTROSCOPE.

§ 14. If a ray of ordinary sunlight be passed through a wedge-shaped piece of glass called a prism, the path of the light will be bent or refracted, and at the same time the light will be separated or dispersed into a series of the most beautiful colours. This series of colours is called the spectrum, a word which simply means an image or likeness (Latin spectrum, an image). When carefully examined the solar spectrum is found to consist of six well-known colours—red, orange, yellow, green, blue, violet. This experiment was first made by Sir Isaac Newton in 1665, who thoroughly investigated the complex nature of light ; but, nearly 150 years previous to this, the astronomer Kepler had first demonstrated its compound nature. We have already seen (§ 2) that the longest visible light waves produce the sensation of red ; while the shortest or more rapid vibrations produce to our eye the sensation we call violet, and between these two extremes are a vast number of intermediate wave-lengths all merging into one another. Thus between the extreme red and the yellow we find many gradual steps, representing bright cherry red, orange red, orange, and orange yellow. Again with a gradual diminution in wave-length the yellow merges into yellow green, then into a pure green, and from green, by a great number of insensible gradations, the light changes to blue, violet blue, and finally to violet. White light, therefore, is composed of a great number of differently coloured rays, distinguishable from one another by the rapidity of their vibrations, or, in other words, by the length of their waves. With.the aid of the prism these rays of light

E

Fic. 1.—Showing refraction of light and production of spectrum by means of the prism.

of different wave-lengths are separated or ‘‘ dispersed,” and spread out in the spectrum in a beautiful series of colours, all arranged according to their respective wave-lengths. The following illustration (Fig. 1) shows how, with an inexpensive glass prism, we might perform this famous experiment of Newton’s for ourselves. (A) represents a slit whereby the light is admitted into a dark room. The beam of light

ANALYSIS OF LIGHT. 13

(B) in passing through the glass prism (C) is bent or refracted out of its course, and in being so, is analysed into its various component rays which go to form the spectrum (D), which can be received on a white screen (KH). If we begin at the top of the spectrum nearest the spot B, which represents the direction the beam of light would have taken had it not been refracted by the prism, we find that the first colour is red, then orange, yellow, green, blue, while the light most refracted is violet.

or,

Fic. 2.—Showing homogeneous nature of spectrum colours: green nos

SSESSSSSSSSSSES STS further decomposed.

§ 15. What is termed the refrangibility of a colour (re, again, and frango, I break in pieces) is the extent to which the colour is refracted or bent in its passage through the prism. Hence red and orange are described as colours of ‘low refrangibility,’ and colours of a ‘‘ high refrangibility,”’ or those ‘belonging to the more refrangible end of the spectrum,” are understood to be the blue and vioiet class.

§ 16. The colours of the spectrum consist of unmixed or homogeneous lights, and they cannot be further decomposed into any other colours. Thus, for example, the green or the violet rays of the spectrum, when passed through another prism, undergo refraction as usual, but they are not decomposed into yellow and blue, or red and blue as we might have imagined ; but remain the same colour, showing that they

Fig. 3.—Simple method of obtaining a spectrum.

are pure and unmixed colours. The various hues seen in everyday life—the dyes and pigments and the colours of flowers, etc.—are never homogeneous, but are mixtures of various differently coloured rays. The foregoing diagram, Fig. 2, shows how the purity of spectrum colours may be shown. The spectrum is thrown upon a cardboard screen similar to that in Fig. 1 and a hole is pierced at the desired part of the spectrum—say, in the green (G)—to separate

OBTAINING A SPECTRUM. 15 the rays. These rays are again passed through a prism (D), when it will be observed that, though the coloured ray is refracted, no other colour is separated. But, when we come to analyse the apparently pure colours of dyes and pigments, we find that they are often very far from being pure, and contain a large number of other coloured rays.

No dye or coloured pigment is known to be absolutely homogeneous in its coloured light.

§ 17. The readiest method of obtaining a spectrum for studying colour is by viewing with the prism a well-illuminated strip of white paper placed on a dark ground— say,Ja piece of black velvet material. Fig. 3 illustrates how

Fic. 4.—Refraction of light.

this is best arranged. The refracting angle of the prism should always be held parallel to the object examined. The dotted lines converging to the eye represent the direction of the rays emerging from the prism, and indicate the locality where the spectrum will appear to the observer. The white strip of paper, on viewing through the prism, will appear coloured the various colours of the spectrum—red, green, and blue. As the rays which traverse a prism are bent, or refracted, away from the refracting angle, as seen in Fig. 4, the object A when viewed through the prism will appear more in the direction of the angle than is really the case, as

BGA’:

In practice it is always considered best to adjust the prism so that the path of the ray of hght A makes equal angles with the two sides at which refraction occurs, thus :—

Fig. 5.—Refracting angle of prism.

§ 18. If we place a blue strip of paper in a line with the white one, and examine them both together in the same manner, we will find the two spectra side by side are not alike.

In the spectrum of the blue will be seen little or no red, orange and yellow rays corresponding to those present in the spectrum of white; while the green, blue and violet rays are similar to that of the white strip, and in exactly the same position. The red, orange and yellow rays have been quenched or absorbed by the blue colouring matter, and as only the green, blue and violet rays are reflected to the eye, the result of their combination gives the sensation of a blue colour. In Fig. 6 (1 and 2) we have a representation of how the spectra of white and blue differ from each other. The shaded parts in the spectrum of No. 2 (the blue) show where the coloured rays are absorbed, and the wnshaded parts show where the colours are reflected.