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88. OPTHALMOSCOPE.

Apparatus. The instrument known as the "Opthalmoscopic Eye of Dr. Perrin,” is admirably adapted as a substitute for a human eye, on which to use the opthalmoscope. It consists of a brass ball, on a stand, representing the globe of the eye, a series of cups, painted to represent different diseases of the retina, which may be inserted in its rear portion, and lenses, representing the cornea and lens, which may be screwed on in front. To these, diaphragms, representing the change in diameter of the pupil, or aperture_in the iris, may be attached. An Argand burner is needed, and a Gräfe's opthalmoscope, also some plates of glass, and a small mirror, with the silvering removed from a circle a quarter of an inch in diameter. This Experiment should be performed in a darkened room, but instead, the light may be cut off by a large screen of black cloth.

Experiment. The opthalmoscope which is used in studying the interior of the eye, has caused a complete revolution in this branch of medical science. Its inventor, Helmholtz, reflected light into the eye, by a piece of plate glass, and then looking through it, found the interior sufficiently illuminated to be visible. .

Take the model of the eye from its box and place it on its stand. Insert the retina marked 1, which represents the normal or healthy retina. Screw on in front the lens marked E. M., and light the burner, placing it by the side of the model eye, and about a foot distant. By reflecting the light into the eye by a plate of glass, and looking through the latter, Helmholtz' experiment may be repeated, and a view of the interior obtained. This is more easily accomplished by using several plates, or better still, with the mirror from which the silvering has been partially removed.

The opthalmoscope consists of a circular, concave mirror, with the silvering removed from the centre. Just behind it is placed a fork, in which either of the five small lenses may be placed. They should be numbered on their edges from 1 to 5, the former being the most concave, the latter the most convex. The retina of the normal eye is placed at a distance from the lens, equal to its principal focus, hence its image is formed at an infinite distance, or the rays emerge parallel. It can therefore be viewed without any lens, using the mirror precisely as in the

previous experiment. The image is better seen, if one of the lenses 1, 2, or 3 is inserted in the fork, as the distance of the image is then diminished, so that the rays diverge, instead of emerging parallel.

This method has the objection of showing only a small portion of the retina at a time, and of bringing the observer too near the eye for convenience. Another method is therefore more commonly used, in which an aerial image is formed in front of the eye, by a convex lens, and this is viewed either directly with the eye, or with a second convex lens of long focus.

Hold the large lens, or objective, two or three inches in front of the model, with the left hand, steadying it with the little finger, which, in the case of the real eye, rests on the forehead of the patient. The mirror should be held a foot or more distant, and turned into such a position as to reflect the light of the gas flame into the model. After a few trials a very beautiful view of the retina will be obtained. The image will be inverted, and may be made as large as the objective by removing the latter to a distance equal to its focal length. To view the other portions of the retina, the model must be turned from side to side, or the patient requested to direct his eye towards various points in turn. The image is improved by placing lens 4 or 5 behind the mirror. In all these experiments, if near-sighted, use a lens with a number lower than that here recommended; thus, instead of 2, use 1, for 5, use 4, etc. The first of the above methods, that is, without the objective, is called the direct, the second the indirect method.

Now screw the larger diaphragm over the lens, and try once more to view the image; then replace it by the small diaphragm, with which it is about as difficult to observe the retina as with the eye in its normal condition. The larger diaphragm corresponds to the case where the pupil is expanded by belladonna. With the small diaphragm it is easier to look a little obliquely into the eye, thus avoiding the light reflected from the lens, which gives bright reflected images of the mirror.

When the eye is near-sighted, or myopic, the retina is beyond the principal focus of the lens. This effect is produced in the model by partly unscrewing the lens, taking care that it does not fall out. An image of the retina is thus formed in front of the

lens, or the rays from it converge. Hence when employing the direct method, a concave lens, as 1 or 2, must be placed behind the mirror, to render the image visible. When the eye is far-sighted, or hypermetropic, and incapable of viewing near objects, the retina is nearer the lens than its focus. Hence the image is formed at a considerable distance behind the lens, and can readily be viewed by the direct method without any lens. This effect is imitated by using the lens marked H, which has a longer focus than the other. The difference is not perceptible by the indirect method, since it is neutralized by a slight motion of the mirror.

Sometimes the cornea has a different curvature in horizontal and vertical planes; it is then said to be astigmatic. The third lens marked A, shows this defect. With this, it is a little difficult to view the retina clearly, since the focus is different in different planes; it will be noticed, however, that the papilla assumes an elliptical, instead of a circular form.

Now replace the emmetropic lens, and insert the various diseased retinas in turn, using the small diaphragm, or, if necessary, the larger one. The retinas are numbered, and represent the following conditions of the eye:

1. Normal retina.

2. Atrophy of the papilla and retina.

3. Atrophy of the choroid.

4. Staphyloma posterior, an old case; blood focus near the macula lutea.

5. Hemorrhage of the retina.

6. Alteration of the retina.

7. Staphyloma posterior. Separation of the retina.

8. Infiltration of the papilla with blood.

9. Exudation of serous fluid, between the choroid and retina.

10. Glaucoma, with the circle of atrophy of the choroid around

the papilla.

11. Glaucoma and hemorrhage of the retina.

12. Atrophy of the papilla, and of the choroid around it.

The papilla is the point of entrance of the optic nerve. The macula lutea, the point of the retina most used.

Atrophy means the gradual wasting away, and absorption of any substance. Staphyloma, a thinning of the covering of the

eyeball, especially around the optic nerve, allowing this portion of the ball to extend backwards. Glaucoma is an increase in quantity of the vitreous humor within the eye, causing a distention of the eyeball, accompanied with acute pain.

89. INTERFERENCE OF LIGHT.

Apparatus. To observe the interference of light, a diffraction bank is employed, which consists of a long horizontal bar divided into millimetres, and carrying sliding uprights, to which the following instruments may be attached, and placed at any desired distance apart. A cylindrical lens to produce a bright line of light, and a brass plate with a slit in it of variable width, like that of a spectroscope. A biprism, or prism of glass with a very obtuse angle, by means of which two closely adjacent images of any object will be formed, and a double mirror designed for the same purpose, whose two halves are inclined at a very small angle, which may be varied by means of adjusting screws. To observe the various effects produced, one of the uprights carries a spider-line micrometer, or a simple eye-piece with cross hairs, which may be moved laterally, and its position determined by a millimetre scale and index. Or, this may be replaced by a small direct vision spectroscope, to analyze any portion of the light passing through the instrument. A screen of ground glass, or paper, may also be substituted for the eye-piece. Although some of the simpler phenomena are visible by ordinary light, yet to obtain the best results sunlight is indispensible. An arrangement is also desirable by which an intense monochromatic light may be obtained, which may be done roughly by interposing colored glasses, but much better by placing a prism in front of the slit, throwing a ray of sunlight through it, and projecting the spectrum thus obtained on the slit. When the day is cloudy, a soda or lithium flame may be employed. A cover should be placed over the whole to cut off the stray light, or a simple piece of black cloth may be employed for the same purpose.

Experiment. According to the Undulatory Theory all space is supposed to be filled with a very rare medium, called ether, whose, vibrations give rise to the phenomena of light. A luminous point throws out concentric spherical waves, whose diameters increase with very great velocity, and each of whose radii is called a ray of light. The direction of the vibrations is transverse, that is, perpendicular to the ray, as is the case of waves of water, and the terms crest and trough are here also used to denote the two opposite positions of any portion of the ether. The distance from

one crest to the next determines the color of the ray, and is called the wave-length. In the same way, the intensity of the light depends. the height of the wave, or distance traversed by each particle. A particle of ether can receive any number of systems of vibrations, whatever their wave-length, intensity, direction, or plane of vibration, and will transmit each precisely as if the others did not exist. If a particle receives two rays of light, precisely similar in every respect, under the influence of both, its motion will be increased, and a more intense light produced. Now suppose one of the rays is retarded by half a wave-length; its crests will coincide with, and neutralize the troughs of the other ray; accordingly the particle will not move at all, and the result will be darkness. The same effect will also be produced if one ray is retarded three, five, or in fact any odd number of half wave-lengths, while if the retardation is an even number of half wave-lengths, crest will fall upon crest, and the light will be increased. This neutralization of one ray by another, or light added to light, producing darkness, is called interference, and by means of it many most important laws have been established.

To produce interference, two precisely similar sources of light are needed, at a very short distance apart. For this purpose, place the cylindrical lens on a support at one end of the diffraction bank, and throw a beam of sunlight through it. A very narrow line of light is thus produced at its focus. Place the biprism on a support, at a short distance in front of it, and two images will be formed very near together, and precisely alike. If, now, a screen is placed near the other end of the bank, its centre will appear bright, since being equidistant from both images it will receive simultaneously the crests and troughs of them both. If, however, a point is taken on one side of the centre, it will be nearer one image than the other, and, accordingly, the crests and troughs will not arrive simultaneously. If, then, the difference of path is an odd number of half wave-lengths, darkness will be produced, while an even number will give brightness.

The consequence will be a series of vertical black bands, corresponding to 1, 3, 5, etc., half wave-lengths. As, however, the light is white, and is composed of rays of all colors, and various wavelengths, the bright and dark spaces will be at different distances

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