Front matter
B 3
THE LIBRARY
OF
Preface
THE author of the following work has been engaged for more than twenty years in teaching the structure, functions, and diseases of the eye, to medical students. His course of lectures on these subjects being limited to a period of three months, or about sixty lectures, and intended chiefly as a practical course on a branch of the healing art, he has generally been obliged to curtail the very interesting matter of the physiology of vision, more especially of late years, in order that he might do that justice to the anatomical and pathological departments of his subject, which the previous studies of his auditors, their ultimate views in attending to the eye, and the continual improvements taking place in practical ophthalmology, seemed to him to demand. In order to supply in some measure the deficiency arising from this necessary abridgment, he has often been solicited to publish a short systematical work on the physiology of vision. As it is now presented to his pupils and the public, the following treatise is one of very humble pretensions. Its chief claim to their indulgence is its being an attempt to explain perspicuously, and in accordance with scientific principles, one of the most interesting subjects of human inquiry, viz.
Vlll PREFACE.
the JawsTof vision. The author is well aware that on such a subject, precise ideas only should be tolerated, and all vagueness rejected; but as unfortunately the study of mathematics is rarely made a preliminary to that of medicine, he has expressed himself as much as possible in common language, and, when mathematical truths were to be delivered, has in general adopted geometrical expressions, which, though more tedious and less elegant than analytical ones, may readily be understood even by those who ai% unacquainted with the elements of Euclid.
It is scarcely necessary to say, that there is little original in the following pages. All that the author has aimed at, has been to present a short and intelligible view of the labours of former inquirers. In doing so, he has availed himself of all the sources within his reach, of the number of which some notion may be formed from the references at the end of the chapters. To the writings of Porterfield, Young, Brewster, Biot, and Miiller, he has been much indebted.
The range of subject embraced in the following pages has been confined by a constant reference to the fact, that they are intended chiefly for the use of those whose business it will be to distinguish and to treat the various diseases to which the eye is liable. At the same time, the author trusts that they will be found to contain such a view of this organ, as may interest also the general reader, for whose benefit he has made it his endeavour to combine the accuracy of a philosophical treatise with the facility of a popular work.
GLASGOW, 2lst April, 1841.
Contents
PACK
Chapter I. — Introductory Observations, 1
1. Function of vision, . . . . . . ib.
2. General account of the optic apparatus, . . ib.
3. Laws of light, . . . . . . .5
4. Explanation of terms, ..... 6
5. General facts respecting reflection and refraction, , . 8
Chapter II. — Rectilineal Progression Of Light, And Formation Of Images By Radiation, . ... . . .10
6. First law of light. Its rectilineal progression, . . ib.
7. Decreasing intensity of direct light, . . . . ib.
8. Formation of images by radiation. Tenuity of light, . 1 1
9. Analogy of the eye to the camera obscura, . . .12
10. Obscurity of the images formed by radiation, . . 13
11. Inversion of the images formed by radiation, . . .14
12. Size of the images formed by radiation, . , . 16
Chapter III. — Refraction Of Light, . . . . .17
13. Third law of light. Its refraction on passing obliquely from
one medium into another of different density. Experiments showing what is meant by refraction, . . . ib.
14. Power of refraction possessed by different substances in different
15. Summary of facts regarding refraction, . . . 21
16. Image of a luminous body rendered smaller and brighter by re-
fraction through a dense medium bounded by parallel planes, ib,
17. By varying the obliquity of the refracting surface, rays can be
refracted in any degree, and to either side, . . 23-
18. Refraction by two surfaces inclined to each other, . . 2"> !*)• f*y giving a determinate figure to the refracting surface, a pencil of rays may be bent into determinate directions, .. £fr
X CONTENTS.
PAGE
20. A curve will bring a pencil of rays to a focus, , . .26
21. Refraction of parallel rays by one convex surface, . . 27
22. Refraction of parallel rays by two convex surfaces, . . 28
23. Formation of images by a double convex lens, « . 29
Chapter IV. — Measurement Op Refraction, . . . .30
24. Refractions not to be measured by the angles of incidence and
refraction, ...... ib.
25. Sines of angles explained. Ratio of the sines different from the
ratio of the angles, . . . . . . ib.
26. Experimental measurement of refraction, . . .31
27. Measurement by means of the sines of the angles of incidence
and refraction. Law of refraction, . . . .32
28. Index of refraction, absolute and relative, ... 34
29. Refractive powers of different substances, . . .35
30. Refractive powers proportionate to the density and inflamma-
bility of bodies, . . . . . .36
Chapter V. — Application Of The Law Of Refraction, . . 37
31. Determination of the course of refracted rays, . . . ib.
32. Geometrical determination of the course of rays refracted at a
plane surface, . . . . . . ib.
33. Geometrical determination of the course of rays refracted at a
curved surface, . . . . . .39
34. Sources of embarrassment in tracing refracted rays by geometri-
cal construction. Change from refraction to reflection at the interior surface of a dense medium. Critical angle, . 40
Chapter VI. — Forms Of Refractive Media, And Their Effects On The Direction Of The Rays Of Light, ... 43
35. Forms of refractive instruments, . . . . ib.
36. Refraction of parallel rays by a homogeneous medium bounded
by parallel planes, . . . . .44
37. Refraction of diverging and converging rays by a homogeneous
medium bounded by parallel planes, . . . .45
38. Refraction by prisms. Experimental measurement of refraction
39. Refractions at spherical surfaces reducible to refractions at plane
40. Refraction by a bent glass, or curved medium with parallel
41. Refraction by a sphere, . . . . .52
42. General facts respecting the axis, optical centre, and classes of
43. Refraction by convergent lenses, . . . .57
44. Refraction by divergent lenses, . . . . ' 59
CONTENTS. XI
PAGE
io. Formation of images by convergent and divergent lenses, . 61
46. Experimental determination of the focal length of lenses, . 68
Chapter VII. — Refractive Powers Of The Lenses Of The Human Eye, 71
47. Lenses of the human eye, . . . . . ib.
48. Dimensions of some parts of the human eye, . . 72
49. Curvatures of the lenses of the human eye, . . 73
50. Refractive densities of the lenses of the human eye, . 77
Chapter VIII. — The Eye Considered As A Dioptric Instrument, . 89
52. Experiments showing inverted images on the retina, . « ib.
54. Optic or visual axis. Axes of cornea and crystalline not coin-
55. Focal centre of the eye. Visual angle. Size of the image.
Apparent magnitude of the object, . . 96
CHAPTER IX. — OPTICAL ABERRATIONS. SPHERICAL ABERRATION. CORREC- TION OF SPHERICAL ABERRATION IN THE EYE, . . .99
56. Three optical aberrations, . . . . ib.
57. Spherical aberration explained, .... 100
58. Spherical aberration modified by certain relations between the
spherical surfaces of lenses, .... 102
59. Spherical aberration obviated by elliptical and hyperbolical
60. Cornea and crystalline supposed to be elliptical or hyperbolical, 104
61. A combination of lenses obviates spherical aberration, . ib.
62. Use of a diaphragm. Aperture of a lens, . . . 105
63. The iris a diaphragm, . . . . . . ib.
64. Increasing density of the crystalline from its periphery in-
65. Summary of the means by which spherical aberration is obviated
66. Experimental proof that spherical aberration is obviated in the
Chapter X. — Chromatic Aberration. Achromatism Of The Eye, . 109
67. Decomposition and dispersion of light explained, . . ib.
68. Newton's discovery of the heterogeneousness of light. Fourth
law of light, . . . . . .110
69. Rays of each particular colour not farther decomposable by re-
fraction, . . . . . . Ill
70. Properties of the solar spectrum. Fraunhofer's fixed lines. In-
dices of refraction for the coloured rays. Mean ray, . 112
71. Recomposition of the prismatic colours into white light, . 113
Xll CONTENTS.
72. Dispersion by lenses. Chromatic aberration, . . .114
73. Correction of chromatic aberration. Dispersive power not pro-
portional to refractive power. Irrationality of dispersion, 116
74. Measurement of dispersive power, . . . , 120
76. Is the eye achromatic ? . . . . . 127
77. Experiments adduced to prove that the dispersion of the eyo is
not corrected, . . . . . .128
78. Does the construction of the eye admit of achromatism ? . 131
79. Does the actual distinctness of vision require the eye to be
achromatic ? . . . . . .133
80. Achromatism of the eye hitherto unexplained, . . 140
Chapter XI. — Distantial Aberration. Adjustment Op The Eye To
81. Distantial aberration explained. Circle of aberration, . ib.
82. Difference of perfect, distinct, and indistinct vision, . .144
83. Effects of size in proportion to distance. Contrast of light and
shade. Simplicity and complexity of objects. Minimum visibile. Images by inflected light. Objects in motion. Ambient darkness, . . . . . .145
84. Nearest and farthest limits of distinct vision. Vision by diver-
ging, parallel, and converging rays, . . .149
85. An adjustment to distances generally admitted ; but denied by
86. De la Hire's doctrine that the sole accommodation consists in
the variation of the pupil. Distinctness of vision aided by contraction and dilatation of the pupil. Vision through a perforated card, . . . . .151
87. Inability of the eye to discern near and distant objects, at the
88. Analogy of the eye to other dioptric instruments, . . 156
89. The presbyopic eye loses the power of accommodation. Fatigue
from viewing near objects, . . . . .157
90. Analogical argument from the vision of diving animals, . ib.
91. Schemer's experiment. Porterfield and Young's optometer, . 158
92. Optical necessity of an adjusting power, . . . 161
93. The eye seeing distinctly at three different distances, the second
of which is about double the first, and the third infinite, as great a change necessary for seeing distinctly at the first and second distance, as at the second and third, . . l(>;j
94. Amount of change necessary to adjust the eye to different
distances, .... 104
Hypotheses formed to account for adjustment, . . 168
Adjustment supposed to be effected by the external muscles of
CONTENTS. Xlll
PAGE
u7. Ramsden arid Home attempt to measure the presumed change
in the curvature of the cornea, . . . . 171
98. Olbers and Young perceive no variation in the image reflected
from the cornea, when the eye is adjusted to different distances, ....... 173
99. Young finds the adjusting power to continue, although the re-
fraction of the cornea is interrupted, . . . 1 74
100. Proofs adduced by Young that no elongation of the axis takes
place, in adjusting the eye to a near object, . . 175
101. Adjusting power lost by extracting the crystalline, . .176
102. Young's proofs of a change of figure of the crystalline, . 178
103. Alleged muscularity of the crystalline, . . .179
104. Adjustment to near objects supposed to be effected by a motion
of the crystalline towards the cornea. Anatomy of the parts at the base of the iris, and surrounding the crystalline. Brewster's experiment on adjustment. Travers's hypothesis. Antagonism of the pupil and ciliary circle. The author's hypothesis, . . . . . . .181
Chapter XII. — Functions Op The Iris. Motions Of The Pupil, . 188
105. Functions of the iris, .... . ib.
106. Iris not the organ of adjustment, . . . .190
107. Size of the pupil does not affect the size of the image, . 191
108. Effects of the form of the pupil, . . . .192
109. Surface of the iris plane. Centres of the iris and pupil not
coincident, ... . .194
110. Natural state of the pupil, . 195
111. Light has no direct effect on the iris. Iris affected by light,
only through the medium of the retina, optic nerve, brain, and third nerve. Motions of the pupil in some cases of complete amaurosis. Consentaneous motions of the pupil of an amaurotic eye with those of the pupil of the sound eye, . ib.
112. Hypotheses regarding the mechanism by which the pupil is
moved. Structure of the iris and uvea. Ciliary nerves. Muscular fibres not detected in the iris. Travers supposes the iris to consist partly of muscular, and partly of elastic tissue. Erectile hypothesis of Mery and Haller. Objections of Fontana and Blumenbach. Contractility of the ciliary nerves observed by Serres, ..... 200
113. Motions of the pupil involuntary, but rendered apparently volun-
tary by an effort at adjustment, ... . . 205
Chapter XIII. — Reflection Of Light By The Eye, . . . 207
114. Second law of light. Its reflection from plane, convex, and
concave surfaces. Spherical aberration of mirrors, . ib.
115. Images reflected from the cornea and crystalline, . .212
XIV CONTENTS.
PAGE
Chapter XIV. — Absorption Op Light In The Etk. Functions Op The Choroid And Pigmentum Nigrum, . . . .214
116. Anatomical relations of the pigmentous membrane. Its analogy
to the rete Malpighianum. It is colourless in albinous animals. Chemical properties of the pigmentum nigrum, ib.
117. Baptista Porta's notions of the eye. Retina transparent.
Absorption of the light which traverses the retina. Opacity
of the iris. Eye of the albino, . . . . 217
118. Reflection of light from the tapetum of some of the lower
CHAPTER XV. — FUNCTIONS or THE RETINA AND OPTIC NERVE, . 222
119. Primitive nervous fibres. Structure of the chiasma. Jacob's
membrane. Nervous and vascular layers of the retina. Entrance of the optic nerve. Transparent point in the vertex of the retina. Microscopical structure of the retina. Size of its papilla), ...... ib.
120. Circulation in the retina generally invisible. Experiments
producing a spectrum of the blood-globules and blood-vessels, 226
121. Retinal images. They are merely a concomitant of vision.
Area of retina. Is it all equally sensible ? Perfect vision effected only in the optic axis. Use of the straight muscles. Duration of impressions on the retina. Comparetti's hypothesis. Experiment illustrative of oblique vision. Extent of oblique vision, ...... 228
122. Mariotte discovers the extremity of the optic nerve to be in-
sensible to light, ...... 232
123. Line of visible direction. Apparent place of an object depends
on the part of the retina impressed, and not on the direction of the incident rays. Porterfield's law of visible direction. Objections to it, . . . . .238
124. Erect vision with inverted images, .... 24 7
Chapter XVI. — Monocular And Binocular Vision. Single Vision With
125. Single vision. Double vision, . . . ib.
126. Explanation of terms. Corresponding or identical points of
the retinae. Horopter. Plane of the horopter, . 253
127. Phenomena of binocular vision, .... 255
128. Theories of single vision with two eyes, . . . 262
129. Cause of vision in relief by dissimilar images on the retinae, . 265
Chapter XVII. — Colours Of External Bodies. Complementary
130. Production of colours by unequal absorption and reflection of
CONTENTS. XV
PAGE
the coloured rays of light. Brewster's analysis of the solar spectrum by absorption, . . . . 267
131. Production of colours by the interference of light. Corpuscular
and undulatory theories of light, . 268
132. Complementary colours. Ocular spectra, » „ 272
Chapter XVIII. — Visual Perceptions, . . . . 2*74
133. Visual perception of figure, ib.
134. Visual perception of place, . .275
135. Visual perception of magnitude, . . 276
137. Visual perception of motion, 280
Chapter XIX. — Vision Aided By Art, . . . 281
138. Images formed by catoptrical and dioptrical instruments, . ib.
139. Effects of divergent and convergent lenses, , . . 282
140. -Vision of myopic eyes aided by concave lenses, and that of pres-
byopic eyes by convex lenses, . . 283
142. Single microscope, ..... ib.
143. Compound microscope, . . . 287
144. Astronomical telescope, . 288
145. Terrestrial telescope, . . 289
146. Galilean telescope. Opera-glass, ib.
147. Reflecting microscopes and telescopes, . . ib.
Chapter XX. — Improvableness Of Vision, 290
148. Increased sensibility of the retina from remaining long in the dark. Difference between the improved and unimproved eye. Herschel's distinction between the magnifying power of telescopes and their power of penetrating into space, . , ib,
THE
PHYSIOLOGY OF VISION.
Chapter I. Introductory Observations
§ 1. Function of vision.
PHYSIOLOGY is the science which explains the functions of the different parts of the living body.
The function of the eye is to distinguish colours ; and by means of the colours reflected or emitted by surrounding objects, to enable us to recognise their presence, forms, sizes, positions, distances, and motions. The immediate instrument of visual perception, light, possessing a diversity of colours, and being presented to the eye in different degrees of intensity, produces particular impressions on the nerve of vision.
§ 2. General account of the optic apparatus.
The eyeball, the immediate organ of vision, is protected by the bones forming a cavity called the orbit. Within this cavity lies the cellular and fatty substance by which the eyeball is supported, and the muscles by which it is moved from side to side, or fixed upon the objects of perception. The blood-vessels which nourish the eyeball, and the nerves with
A
OPTIC APPARATUS.
which its different parts are furnished, are also contained within the orbit.
The inside of the eyelids, and the anterior 5-12ths of the eyeball, are covered by a mucous membrane called the tunica conjunctiva, upon the exterior surface of which the tears are poured out from the lacrymal gland. The mucus of the conjunctiva serves to lubricate the parts, and render their motions easy ; the tears wash away that mucus, as well as such foreign particles as may alight on the surface of the eye.
The eyelids, fringed with the eyelashes, and surmounted by the eyebrows, are opened and shut by particular muscular forces, and complete the parts destined to protect the eyeball — the tutamina oculi.
Although it is probable that none of its surfaces are truly spherical, the human eyeball is generally described as formed by two unequal spherical segments. The spheres to which these segments belong, would, if continued, touch each other internally at c, fig. 1. The one is part of a small sphere ; and the other, part of a larger. The diameter of the larger sphere, 1, fig. 1. measures about 19-20ths, and that of the smaller sphere, 2, 13-20ths of an inch.
The diameter of the base of the segment of the smaller sphere, a b, measures 9-20ths of an inch.
The external measurement of the axis of the eyeball, c d, fig. 2. is, in general, equal to its transverse diameter, ef. The eyeball, therefore, is spherical, except where the two segments of which it is composed are connected by a portion coincident with neither. The larger segment, or Fig. 2.
sclerotica, runs gradually into the smaller segment, or cornea ; each suffers a slight alteration of form in being united, and thus an annular depression, a b, is created at their junction.
The eyeball consists of many parts, and these present a
Fig. 1
OPTIC APPARATUS. o
variety of different textures. Its construction, generally considered, is that of several concentric spherical membranes, closely applied to one another, within which are contained certain transparent media, of different densities.
Its external shell is formed posteriorly by an opaque fibrous membrane, the sclerotica, 1, fig. 3. already referred to ; while framed into the front of €the eyeball is the transparent cornea, 2. In a horizontal section of the eyeball, such as is represented in fig. 3. 5-6ths of the circumference of the section are formed by the sclerotica, and the remaining 6th by the cornea.
Within the sclerotica lies the choroid coat, 3, lined by a pigment of a dark brown colour.
While the greater part of the choroid invests the retina, 4, its anterior portion is in contact with the vitreous humour, 5. In a horizontal section, the portion in contact with the vitreous humour, and which receives the name of the ciliary ring, or corpus ciliare, measures about one-fifth of the whole. Around the crystalline body, the ciliary ring terminates in about seventy plaits or folds, 6, called the ciliary processes.
If we look through the cornea into the interior of the eye, we observe a membranous disc, called the iris, 7, nearly coinciding with the common base of the two segments of which the shell of the eye is formed. Nearly in its centre, the iris presents an aperture of variable diameter, known by the name of the pupil, contracting and expanding in the living subject, according to the brightness of the light, and the distance of the object to which the eye is directed. The anterior surface of the iris has a striated appearance, and is generally of a bluish or hazel colour; its posterior surface, like the internal surface of the choroid, is covered with dark-brown pigment.
Within the choroid, is the retina, 4, a transparent nervous membrane, extending from the optic nerve, 8, at the back of the eye, to within one-fourth of an inch of the circumference
4 OPTIC APPARATUS.
of the crystalline lens, 9, and forming a cup, of which a horizontal section measures more than two-thirds of the circumference of a circle.
The globular space within the concavity of the retina is occupied by one of the transparent media of the eye, called the vitreous humour, 5. It is a fluid, so supported by a cellular structure as to present a gelatinous degree of consistence. Between the termination of the retina and the edge of the crystalline body, the vitreous humour is covered by the ciliary ring or corpus ciliareoi the choroid, including the ciliary processes, already mentioned. This ring measures about one-fourth of an inch in breadth, but is somewhat broader on the temporal than on the nasal side of the eye.
Imbedded in the front of the vitreous humour, and close behind the pupil, lies the crystalline lens, 9, enclosed within a peculiar membrane or capsule.
The space between the crystalline and the cornea is occupied by a fluid called the aqueous humour. The iris, supported on both sides by this fluid, partially divides the cell in which it is contained into two compartments, known by the names of the posterior, 10, and anterior, 11, chambers, which communicate by the pupil.
Such is a general enumeration of the parts of which the eyeball consists. For an account of their structure and connexions, recourse must be had to books of anatomy.1 In reference to their functions, the different parts of the eyeball may be arranged in four classes, viz. 1. The external shell, or consolidative coat,2 formed by the sclerotica and cornea. 2. The dioptric parts, i. e. refractive media, or lenses ; namely, the vitreous, crystalline, and aqueous humours, along with which must again be reckoned the cornea, as it performs a double office, serving at once as a lens and as portion of the consolidative coat of the eye. 3. Parts subsidiary to the perfection of the eye as an optical instrument ; namely, the choroid, which serves to absorb the rays of light, and the iris, which is a diaphragm for obviating the spherical aberration of the lenses. 4. The specially sensitive parts; namely, the retina, and the extremity of the optic nerve, with which the retina is continuous.
LAWS OF LIGHT. D
The optic nerve, 8, quits the retina and the eye by an aperture in the posterior part of the choroid and sclerotica, not in the axis, but about one-fifth of an inch to the nasal side of the axis, and a little above the equator3 of the eyeball. It passes back, through the orbit, and through a hole in the sphenoid bone, into the interior of the cranium, There, at the distance of about an inch and three quarters behind the eyeball, the nerve from the right eye meets with that from the left. The two nerves mingle, or partially decussate their fibres ; then separate, proceed under the hemispheres of the cerebrum, traverse and adhere to the crura cerebri, embrace the tubercles called corpora geniculata externa, communicate with the thalami nervorum opticorwn, and are supposed to end in the corpora quadrigemina, a little behind the middle of the brain.
§ 3. Laws of light.
It is unnecessary, on this occasion, to enter on any inquiry concerning the nature of light ; for the theory of vision rests upon observations, totally independent of that question. So far as our subject is concerned, it matters not whether the change that occurs in space previous to the sensation of vision, be the progress of a succession of material particles, or of a vibratory movement of a line of other particles.
The facts upon which the theory of vision is founded are of so general an application, in regard to the phenomena of light, and the construction of optical instruments, that they are known by the name of the laws of light* They are principally these four : —
1. That from every luminous point, light tends to radiate in every direction, in straight lines.
2. That falling upon certain surfaces, light is reflected from them ; and, in those cases, the angle of reflection is always equal to the angle of incidence.
3. That on passing obliquely out of one transparent medium into another of different density, light proceeds no longer in the same straight line, but is bent or refracted.
0 EXPLANATION OF TERMS.
4. That light consists of differently coloured rays, possessing different degrees of refrangibility.
From these few principles, established by experience, a vast multitude of truths, equally certain with the principles themselves, and in fact, the whole theory of vision may be deduced by the mere application of mathematical reasoning. The first of the laws of light is the foundation of optics proper, or the theory of direct light ; the second is the foundation of catoptrics, or the theory of reflected light ; the third of dioptrics, or the theory of refracted light ; and the fourth, of chromatics, or the theory of colours.
The above is the order in which the laws of light are commonly enumerated ; but, from the eye being a dioptric instrument, and its catoptric effects being merely incidental, we shall find it more convenient to take them up in the order of 1,3,4,2.
§ 4. Explanation of terms.
1. Any transparent body, through which light passes, as air, water, glass, &c. is called a medium. Even empty space is considered a medium.
2. The least portion of light, which emanates from a luminous body, is called a ray. Although a ray of light, regarded physically, is an infinitesimal pyramid, having for its vertex a luminous point, and for its base an infinitely small portion of any surface illuminated by it, rays are represented, for the sake of convenience, by mere mathematical lines, drawn in the directions in which the light is supposed to move. ,
3. A slender portion of rays, separated from the rest, is called a pencil, and a greater quantity, a beam of light.
4. Pencils of rays are generally of a conical form. Rays emanating from a point, as R, fig. 4, and receding from each
Fig, 4.
EXPLANATION OF TERMS.
other as they advance, are called diverging rays ; tending to a point, as F, fig. 5, at which they at last unite, or would
Fig. 5.
unite, if not prevented, they are styled converging rays.
Rays of light are naturally divergent, but they are made to converge artificially, by being reflected or refracted.
A pencil of parallel rays, fig. 6, consists in such as proceed
Fig. 6.
through all their course at equal distances from one another.
5. The point from which the rays of a pencil diverge, as R, fig. 4, or towards which they converge, as F, fig. 5, is called their focus* or focal point. Any point from which rays proceed is also called a radiant or radiating point, or simply a radiant.
The point from which rays are made apparently to diverge, or towards which they are made apparently to converge, by reflection or refraction, when in reality they diverge from or converge towards another point, is called a virtual focus.
The point to which parallel rays are reflected or refracted, receives the name of the principal focus.
The distance of the focus from the reflecting or refracting surface is called the focal distance, or focal length. This generally refers to the principal focus.
The focus before reflection or refraction is called the focus of incident rays ; and the focus after reflection or refraction, the focus of reflected or refracted rays. Both together are called conjugate foci ; and they are so related, that if either of them be the radiant point, or the focus of incident rays, the other will be the focus of reflected or refracted rays.
6. When a ray of light, falling or incident upon any surface,
8 REFLECTION AND REFRACTION.
is turned back into the medium in which it was moving, it is said to be reflected.
7. When a ray of light passes out of one medium into another, and has its direction changed at the common surface of the two media, it is said to be refracted.
8. The angle contained between the incident ray and a line drawn perpendicular to the reflecting or refracting surface, or to a plane touching that surface, at the point of incidence, is called the angle of incidence.
9. The angle contained between the reflected ray and the perpendicular to the reflecting surface at the point of incidence, is called the angle of reflection.
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