Practical remarks on near sight, aged sight, and impaired vision : with observations upon the use of glasses, and on artificial light

Front matter

dbarles a. ©liver.

NEW MEDICAL WORKS,

PUBLISHED BY

Chapter I

Light — Theories of — Reflection — Refraction — Velocity — Composition of — Solar Spectrum — Colours — Calorific rays — Chemical rays — Table of Undulations— Interference of light — Fraiinhofer's Lines — Human Eye — Form — Structure — Cornea—Aqueous membrane — Conjunctiva — Aqueous humour — Crystalline lens — Vitreous humour — Sclerotica — Choroid — Pigmentum nigrum — Tapetum — Ciliary processes — Ciliary ligament — Iris — Refractions in the Eye — Retina — Nerves — Inverted image — Single vision— Idea of solidity . . Page 1

Chapter II

Myopia (near sight) — Varieties — First form — Causes — Effects — Symptoms — Hydrophthalmia — Conical Cornea — Imperfect cataract — Cases— Treatment — Glasses — General treatment— Second form — Power of adjustment — Symptoms — Cases — Remarks . . . . .39

Chapter III

Presbyopia — (aged sight) — Causes — Symptoms— Cataf act — Amaurosis — Occurrence in youth — Cases — Treatment — u Preservers"—Glasses — Injurious effects of overworking the eyes —Advice as to light — African travellers — Esquimaux — Sir W. Herschel — Presbyopic Glasses— Cautions as to their use . 76

Chapter IV

Lenses — Forms of — Action of — Convex lens — Concave lens — Spectacles — Invention of— Manufacture of — Pebbles— Neutral tint glasses — Reading glasses — Single eye glasses — Periscopic glasses — Lenses of unusual form — Selection of glasses — Cheap spectacles — Advertisements — Imperfect glasses — Spectacle

CONTENTS. IX frames — steel — gold — silver — tortoiseshell — Cleaning spectacles— Table of focal lengths — Eye-pieces — oval — circular — Modes of ascertaining focal lengths of lenses— Cataract glasses — Lenses for Divers . . . 108

Chapter V

Effects of Artificial Light — Irritation of the eyes — Chronic ophthalmia — Symptoms — Appearances — Impaired vision — symptoms — Persons subject to — Cases — Milton's description of his own case — Treatment of chronic ophthalmia — Scarification— Blisters — Ointments — Collyria — Treatment of impaired vision — Rest to the eyes — Injurious effects of crowded workrooms — Cupping — Counter-irritation — Mercury — Tonics — Cold ablution— Diet . . . . 136

Chapter VI

Artificial Light — excess of red and yellow rays — Combustion — Effects on the retina — Objections to— Carbonic acid gas — Impurity of certain atmospheres — Arsenic — Sulphur — Chlorine — Sulphuretted hydrogen — Carburetted hydrogen — Gas light — Gas burners — Argand — Camphine lamp — Naphthalized coal gas — Oil lamps — Hot oil lamp — Solar lamp — Monochromatic lamp — Wax candles — Tallow candles — Composition candles — Modes of obviating injurious effects of Artificial Light— Damping the atmosphere — Silken shades — Coloured spectacles — Paper shade — Metallic shade — Ground glass shades — Vitiation of the atmosphere — Ventilation — Faraday's ventilator — De la Garde's ventilator 162

Appendix, page 186.

A. Colourblindness.

B. Effects of the non-luminous rays upon the eye.

Mode in which the adjustment to distance is performed.

C. Effects of SnowBlindness, and modes of obviating them. Experiments by Sir Isaac Newton upon luminous impressions on the retina.

D. Reflections by Milton upon his blindness— Particulars of Galileo's blindness.

The Frontispiece, and Fig. 23, are (with some modifications) reduced from the excellent plate by Dr. Wharton Jones, in Dr. Mackenzie's Treatise on the Diseases of the Eye. They are intended to represent the parts displayed in a vertical section of the eye.

1. Conjunctiva covering the cornea.

2. Cornea.

3. Sclerotica.

4. Membrane of the aqueous humour.

5. Choroid.

6. Retina.

7. Ciliary processes.

8. Iris.

9- Crystalline lens.

10. Vitreous humour.

11. Optic nerve.

12. Anterior chamber.

13. Posterior chamber.

PRACTICAL REMAKES.

CHAPTEB I.

LIGHT.-ANATOMY AND PHYSIOLOGY.

SECTION I. Of Light

Before entering upon the consideration of the physiology of the eye, it will be requisite briefly to state the chief facts connected with light, and the laws by which it is governed.

Two theories have been entertained respecting the nature of light. Newton imagined that there was a material substance emitted by all self-luminous bodies in extremely subtle particles, and moving in straight lines with great velocity, which, impinging upon the optic nerves, produced the sensation of light. Of late years, however, another theory has gained ground, and is now al-

B

OF LIGHT.

most universally adopted. It is to the effect that all space is filled with an extremely elastic and rare ether, and that light is the result of undulatory movements communicated to this ether by selfluminous bodies ; which undulatory movements being transmitted to the optic nerves, give rise to the sensation of light.

When a ray of light impinging upon a surface is thrown back into the medium in which it was moving, it is said to be reflected, and in such case the angle of incidence and the angle of reflection are always equal.

Fig. l.

Let a d (Fig. 1) be a ray of light falling upon a plane speculum, m n ; at the point D, it will be reflected in the direction d b ; and the angle b d e will be equal to the angle ade,

The ray A D is called the incident ray, and b d the reflected ray ; ade the angle of incidence, and B D E the angle of reflection ; and a plane passing through A D and B D, or the plane in which these two lines lie, is called the plane of incidence , or the plane of rejlectioii.

Light consists of rays of different colours and of different amounts of refrangibility.

By a ray of light is meant the thinnest imaginable stream emanating from a luminous body. Rays which recede from each other as they advance are called diverging rays. Those which approach each other as they proceed are termed converging rays.

The point to which rays converge is called theiv focus, or focal point.

That from which they proceed, their radiant or radiating point.

The distance of the focus from the reflecting or refracting surface is termed the focal distance or focal length.

The point from which rays, not originally parallel, appear to diverge, or towards which they appear to converge, whether from reflection or refraction, is called a virtual focus.

The point to which parallel rays are reflected or refracted, is denominated the principal focus.

When a ray of light passes out of one medium into another, and has its direction changed at

b 2

OF LIGHT.

the common surface of the two media, it is said to be infracted.

Fig. 2.

Let a B (Fig. 2) represent a ray of light passing through air, and incident obliquely on the surface of water at b. Instead of pursuing its original course to c, it will be refracted in the direction b d, and drawn towards p R, a line perpendicular to the surface of the water at the point of incidence b. If the dense medium is bounded by plane surfaces, parallel to each other as represented ; on quitting the dense medium, to enter one which is less dense, as air, the ray will undergo a second and opposite refraction. The ray b d. quitting obliquely the second surface of the dense medium, is refracted from the perpendicular p' r', and takes the direction D e, which is parallel to c f, the original course of the ray. Light, then, passing obliquely from a rare into a dense medium,* as from air into glass, is bent or refracted from its course totvards a line perpendicular to that point of the denser surface where the light enters it. But where it passes obliquely from a dense into a rare medium, as from glass into air, it is refracted in a direction from that perpendicular.

Eefraction is always accompanied by some degree of reflection. There is no substance known which is perfectly opake or perfectly transparent ; charcoal, the most opake of bodies, in a different state of aggregation as diamond, is one of the most perfectly transparent. Gold may be beaten so thin, as to be pervious to light, and the purest air or clearest water gradually extinguishes by absorption, the rays which are transmitted through it. Thus it is, that objects at the bottom of very deep water are

* The refracting power and the density of a substance, do not always travel together. The diamond has greater refracting power than some other substances more dense.

C OF LIGHT.

invisible, and more stars are seen from the summit of a very lofty mountain than from a less altitude ; because the light from the most distant stars becomes so much enfeebled by its passage through the lower atmospheric strata, that it has not sufficient power to affect the sight.

Waves of light, (like those of sound,) are transmitted in every direction from a luminous body, and their intensity is inversely as the square of the distance. The velocity of light (or more strictly speaking, of the luminous undulations) is about a hundred and ninety-two thousand miles in a second. Although the progressive motion of light is uniform and in a straight line, the vibrations of the particles are always at right angles to the direction of the line.

. The compound nature of white light was proved by Sir Isaac Newton, by passing a sunbeam through a prism, which separating the rays by refraction, gave rise to the formation of a spectrum consisting of seven colours — viz. red, orange, yellow, green, blue, indigo,, and violet, all possessed of different degrees of refrangibility ; the red being least refrangible, the violet the most so. Finding by experiments that each coloured ray was incapable of further decomposition, Sir Isaac concluded that white light consisted of seven kinds of homogeneous light. Sir David Brewster, however, has proved by a variety of ingenious experiments that the solar spectrum consists of three primary colours, red, yellow, and blue, each of which exists throughout the whole extent of the spectrum, but with different degrees of intensity in different parts ; and that the super-position of these three produces all the seven hues according as each primary is in excess or deficient.

The proportion of each colour in the spectrum, according to the observations of Newton and of Fraiinhofer, is as follows :

Newton. Fraiinhofer.

Orange 27 . 27

Yellow 40 27

Green 60 46

Blue 60 48

Indigo 48 47

Violet 80 109

360 360

Besides the seven colours ordinarily described, it would appear from Sir J. Herschell's experiments, that there is just beyond the violet a band

? OF LIGHT.

of coloured light of still greater refrangibility ; it has been denominated the Lavender band.

Colours are frequently supposed to be certain properties inherent in substances.* In reality, colour is not a property of matter, but arises from the action of matter upon light, and depends simply upon such an arrangement of the constituent particles, that certain of the coloured rays which help to form white light, are reflected, whilst the others are absorbed or transmitted.

Bodies that reflect all the rays appear white, those that absorb all, black, but the generality of substances absorb some and reflect others. Scarlet cloth absorbs almost all the colours, except the red, which it reflects. In like manner, blue cloth reflects the blue rays, and absorbs the rest.

The colour of transparent substances depends upon their property of absorbing some of the colours of white light, and transmitting others. The rich dark light transmitted by blue smalt glass is composed of all the colours of white light, minus those which have been absorbed by the glass. All tints are the most brilliant when viewed in light of their own colour ; for instance, the hue of scarlet cloth is infinitely more intense

* Appendix A.

when viewed in red light, and a blue ribbon is much more vivid in blue light than in white.

Sir W. Herschell discovered that there are rays in the solar spectrum which give rise to the sensation of heat, independently of those of light. Perhaps their vibrations are not of sufficient velocity to communicate to the retina the sensation of light; so that it is only by their calorific properties that we are cognisant of them.* These calorific rays are most abundant a little beyond the red extremity of the spectrum, and gradually diminish towards the violet, beyond which they are imperceptible.

Dr. Wollaston made the discovery that rays known only by their chemical action, exist beyond the extreme violet : it is probable that the rapidity of the undulations of these rays is too great to make an impression upon the retina, so that it is by their chemical effects alone that we are made acquainted with their existence : these effects are seen in the fading of vegetable colours and the blackening of nitrate of silver. It has been demonstrated beyond a doubt, that the coloured rays in the solar spectrum differ materially, not only in their refrangibility, but also

* Appendix B.

B 5

OF LIGHT.

in the length and rapidity of their undulations. The following table, which has been most carefully constructed by Sir. J. Hershell, shows this. The subject is of much importance, and I shall have occasion to revert to it again.

Coloured Rays.

Length of luminous waves in parts of an inch.

Number of undulations in an inch.

Number of

undulations in a

second.

458 mills, of mills

Extreme Red

Red Intermediate

Orange Intermediate

Yellow Intermediate

Green Intermediate

Blue Intermediate

Indigo Intermediate

Violet Extreme Violet

When two equal rays of light, proceeding from two neighbouring points, fall upon a sheet of white paper, they produce a spot which will be twice as bright as either ray would produce singly, provided the difference in the lengths of the two rays is the same. A similar effect takes place if the length is twice, three times, or four times any given distance ; but if the rays interfere at intermediate points, that is, if the difference be equal to one half, or one and a half, two and a half, three and a half that distance, the one light will extinguish the other and produce a dark spot. This is called the interference of light, and is analogous to the beats which arise from two musical sounds nearly in unison with each other; the beats taking place when the effect of the two sounds is equal to the sum of their respective intensities, corresponding to the luminous spots where the effect of the two lights is equal to the sum of their separate intensities; and the cessation of sound between the beats, when the two sounds destroy each other, corresponding to the dark spots, where the two lights produce darkness. The colours of grooved surfaces, mother of pearl, &c, may be explained by the doctrine of the interference of light.

If a small sunbeam be transmitted through a perfect prism, and the spectrum be received on a sheet of white paper, it will present the appearance of a ribbon shaded with all the prismatic colours, and striped across its breadth by numerous dark lines : these were discovered simultaneously by Wollaston and Fraunhofer, and are alwTays found in the same parts of the spectrum, and maintain the same relative positions, breadths, and intensities. The light from the sun, whether direct or indirect, has its own series of lines ; and that derived from the planets, and from the combustion of substances, also presents similar lines, but arranged differently in each spectrum. The spectrum from the light of a lamp contains none of the dark fixed lines seen in the solar spectrum. Electrical light, when analysed by the prism, presents very different appearances to the solar light : Eraiinhofer found that, instead of the fixed dark lines of the solar spectrum, the spectrum of an electric spark was crossed by numerous bright lines, which (according to Professor Wheatst one) differ in number and position, with the metal from which the spark is taken.

The practical value of Eraunhofer's lines is considerable, as they present a standard of comparison, by which measures can be taken of the refractive and dispersive powers of bodies.