first moistened with cold water, and boiling water then poured upon it; if it does not come thick enough it may be heated for a moment to the boiling point, but should not be boiled, otherwise it becomes watery and loses its adhesiveness. To prevent the cardboard from curling up, it is well to moisten it before attaching the photograph. The whole should then be passed through heavy rollers to give a good finish to the picture.

87. TESTING THE EYE.

Apparatus. A set of test-types, or letters of various sizes, should be placed at distances from the student proportional to their size. On the table are placed the optometers described below, a reading microscope on a stand, for the experiment of Cramer and Helmholtz, a gas flame, tests for astigmatism, and a set of concave, convex and cylindrical lenses of various curvatures, and prisms of various angles.

Experiment. The eye is formed like a camera obscura, in which the retina takes the place of the screen on which the image is received. In front of the lens is a delicate curtain, called the iris, which gives to the eye its color, and in this is a circular hole, the pupil. The iris is formed of fibres, some circular, others radial, the contraction of the first diminishing, of the second increasing the size of the pupil, and hence the amount of light admitted into the eye. These changes are readily seen by covering the eye with the hand, removing the latter, and looking in a mirror; the pupil will then be seen to contract slowly, having dilated in the dark. The pupil of the other eye will also contract a little, as they both commonly act together.

The image of objects at various distances, may be brought to a focus on the retina by varying the form of the lens, while in the camera it is effected by varying its position. By this change which is called accommodation, objects may be seen with perfect distinctness, with a normal or perfect eye, at any distance, from about 4 inches to infinity. Call P the nearer distance, and R the farther, for any eye, then 11=1/ is called the range of accommodation, and is much employed in studying defects of the In the case of the normal eye, the range of accommodation

eye.

1 Α P R

evidently equals 1. The most common defect to which the eye is subject, is that the ball is not spherical. If it is elongated, the retina is carried too far off, and objects must be brought nearer the eye, to render them distinctly visible. Such an eye is called myopic, or near-sighted. If the ball is flattened, near objects cannot be easily seen, and the eye is then hypermetropic, or farsighted. This must not be confounded with the effect of age, which renders the lens harder and thus diminishes the range of accommodation, so that distant objects alone can be seen. The eye is then said to be presbyopic. The normal eye is called emmetropic.

To measure the far and near points optometers are used. One of the simplest of these consists of a board on which a straight line is ruled. At one end is a sheet of metal, with two fine slits very near together. The eye is placed close to the slits, so as to look through both, when it will be noticed, that the nearer end of the line appears double, since the images formed by the two slits cannot be brought together by the eye, on account of the short distance. Sometimes, also, the farther end will appear double, if the eye is myopic. The points, where the line divides, give the far and near limits of distinct vision. A better form of optometer, resembles the apparatus represented in Fig. 60, only it is much smaller. The lens, which should have a focus of 6 inches, is fixed at the end of the rod, in the place of the gas-burner, A, and some very fine print, or other minute object, is attached to the screen, C. Now measure the greatest and least distance at which the print can be read, when the eye is placed near the lens. Call 1

1

1

these distances P' and R'. Then

1 6'

1

and

R R

1

6'

near distances of accom

which gives P and R, the far and

modation. For the normal eye, as P equal 2.67, and R′ = 6.

10, R = ∞, P' should

Another excellent form of optometer is very simply made of a sheet of cardboard or brass. This is pierced with three sets of holes, the first a single hole 1 mm. in diameter, the second several smaller holes near together, for instance three rows of three each at intervals of a millimetre, and thirdly two holes 3 mm. apart,

one of which may be covered with a plate of red glass. View a small distant point of light, as a candle or gas flame, through them, and the appearance will vary according as the eye is normal, far or near sighted. Looking through the single hole and moving the card rapidly from side to side, the light will appear to remain stationary, if the eye is normal, otherwise it will appear to move as the rays pass through different portions of the pupil. In the same way the nine holes will give nine images, if the eye is not normal. If the two holes are used, two circles are seen which overlap as the card is brought near the eye. If the eye is not normal two images will be formed in the overlapping part, since the rays falling on different parts of the lens are not brought together to the same point on the retina. Now cover the right hand hole with the red glass, and if the eye is far-sighted the left hand one will be colored, if near-sighted the right one, since in the latter case the rays cross, coming to a focus before reaching the retina. From the distance between the images, the amount of the defect may be measured. Thus bring a second candle flame near the first, until two of the images overlap, forming three instead of four; the distance between the candles then equals the interval between the images, and from it the lens required to render the eye normal, may be determined. Let D be the interval between the images, d that between the two holes, and B the distance from the light to the eye. Then, D: d= B:ƒ in which f is the focal length of the lens required to produce distinct vision. By turning the card the two images will appear to revolve around each other, and if the eye is astigmatic their distance apart will vary. If the eye is normal all these effects may still be observed by putting on convex, concave, or cylindrical glasses. When observing one's own eye it is often more convenient to view the reflection of two lights near at hand in a distant mirror, so that their distance apart may be more easily varied.

The best test, however, for the eye, is to see if all the test-letters can be read easily. To understand how objects look to a nearsighted person, put on a pair of convex glasses, and repeat these observations with them. Do the same with concave glasses, which give the effect of hypermetropia. See also if the foci of the glasses can be determined correctly from these observations.

Another defect present in some eyes is astigmatism, or unequal focus for horizontal and vertical lines. For example, the eye may be normal for vertical, and near-sighted for horizontal lines. It is detected by looking at a test made of several series of strongly marked equidistant lines, running in various directions. This defect is corrected by using cylindrical lenses, or if, as often happens, the eye is myopic or hypermetropic at the same time, by means of lenses cylindrical on one side, and convex or concave on the other. Many persons who could never see well with common glasses, experience wonderful relief from such lenses. Sometimes. the axes of the eyes are not quite parallel, a defect remedied by the aid of prisms, with very acute angles.

Λ

V

V

Many theories have been advanced to explain accommodation, some supposing that the retina was drawn back, others, that the lens moved, and others, that the ball of the eye changed its shape. The true explanation is deduced from the following experiment, devised and worked out quantitively, by Kramer and Helmholtz. Two persons are required; one, whose eye is to be examined, sits facing a candle, or gas-burner, while the other examines with the reading microscope the reflection of the light in his eye. Three images will be seen, as shown in Fig. 62, in which V is intended to represent the reflection of the candle flame. The eye being directed towards a distant ob

Fig. 62.

Λ

V

Fig. 63.

V

ject, the first image to the right is formed by reflection in the cornea, or front surface of the eye. It is bright and upright, as the surface is convex. The second is formed' by the front surface of the lens. It is much fainter and larger, but also upright. The third being formed in the posterior and

concave surface of the lens, is minute and inverted. Now let the eye be directed towards a near object. The first and third images will remain unchanged both in size and position, showing that the cornea and rear surface of the lens are not altered, either in position or curvature. But the second image, as shown in Fig.

63, approaches the first, and diminishes in size, showing that the front surface of the lens is pushed forward, and becomes more curved. Measurements also show, that the amount of the change is just sufficient to account for the required difference in focus. This experiment is very conclusive, as each of the other hypotheses is disproved by it. If the cornea altered, the first image only should move. If the lens moved, the second and third images should approach the first without altering their size, and if the form of the ball altered, the relative position of all three should remain unchanged.

All parts of the retina are not equally sensitive; although the eye can perceive objects through an angle of about 150° horizontally, and 120° vertically, yet the portion where vision is most distinct is quite small, not more than 3° or 4° in diameter. This portion of the retina, which is called the macula lutea, is used almost exclusively whenever objects are carefully examined, and probably on this account, is not quite as sensitive to very faint objects as the adjacent parts. At any rate, it is very customary with astronomers, when trying to see very faint objects, to direct the eye a short distance from their supposed place, and try to catch sight of them, when not in the centre of the field of view. A short distance from the macula lutea, on the side towards the nose, is a small circle where the optic nerve enters. This space, although so near the most sensitive portion of the retina, is totally insensible to light. It is called the papilla, or sometimes the punctum cœcum, or blind spot. To observe it, mark two points on a sheet of paper, about 4 inches apart, and closing the left eye, direct the other to the left hand point, and then moving the paper to and fro, a certain distance will be found, at which the other point will completely disappear. By using two lights, this experiment may be rendered still more striking, as even a bright light may be made to completely disappear, although objects all around it are visible.

A great variety of experiments may be made, depending on the stereoscopic effects obtained with two eyes, or on the persistence of vision, using such instruments as the thaumatrope, chromatrope, and phenakistascope.