EYE 819 to a certain extent, the action of the other ; and (4) the structure of the lens is such that its power of refraction diminishes from the centre to the circumference, and conse quently the rays farthest from the axis are less refracted. (b) Astigmatism. -Another defect of the eye is due to different meridians having different degrees of curvature. This defect is known as astigmatism. It may be thus de tected. Draw on a sheet of white paper a vertical and a horizontal line with ink, crossing at a right angle; at the point of distinct vision, it will be found impossible to see the lines with equal distinctness at the same time: to see the horizontal line distinctly the paper must be brought near the eye, and removed from it to see the verti cal. In the cornea the vertical meridian has a shorter radius of curvature, and is consequently more refractive than the horizontal. The meridians of the lens may also vary ; but, as a rule, the asymmetry of the cornea is greater than that of the lens. The optical explanation of the defect will be understood with the aid of fix. 8, FIG. 8. Diagram illustrating Astigmatism. Thus, suppose the vertical meridian C A D to be more strongly curved than the horizontal F A E, the rays which fall on C A D will be brought to a focus G, and those fall ing on F A E at B. If we divide the pencil of rays at suc cessive points, G, H, I, K, B, by a section perpendicular to A B, the various forms it would present at these points are seen in the figures underneath, so that if the eye were placed at G, it would see a horizontal line a a if at H, an ellipse with the bng axis a a parallel to A B; if at I, a circle ; if at K, an ellipse, with the long axis, b c, at right angles to A B; and if at B, a vertical line b c. The degree of astigmatism is ascertained by measuring the difference of refraction in the two chief meridians ; and the defect is corrected by the use of cylindrical glasses, the curvature of which, added to that of the minimum meridian, makes its focal length equal to that of the maximum meridian. (c) Aberration of Ref Tangibility. When a ray of white light traverses on a lens, the different rays composing it, being unequally refrangible, are dispersed : the violet rays (see fig. 9), the most refrangible, are brought to a focus at FIG. 9. Diagram illustrating the Dispersion of Light by a Lens. e, and the red rays, less refrangible, at <7. If a screen were placed at e, a series of concentric coloured circles would be formed, the central being of a violet, and the circumference of a red colour. The reverse effect would be produced if the screen were placed at d. Imagine the retina in place of the screen in the two positions, the sensational effects would be those just mentioned. Under ordinary circum stances, the error of refrangibility due to the optical con struction of the eye is not observed, as for vision at near distances the interval between the focal point of the red and violet rays is very small. If, however, we look at a candle flame through a bit of cobalt blue glass, which trans mits only the red and blue rays, the flame may appear violet surrounded by blue, or blue surrounded by violet, according as we have accommodated the eye for different distances. Red surfaces always appear nearer than violet surfaces situated in the same plane, because the eye has to be accommodated more for the red than for the violet, and consequently we imagine them to be nearer. Again, if we contemplate red letters or designs on a violet ground the eye soon becomes fatigued, and the designs may appear to move. (d) Defects due to Opacities, &c., in the Transparent Media. When small opaque particles exist in the transparent media, they may cast their shadow on the retina so as to give rise to images which are projected outwards by the mind into space, and thus appear to exist outside of the body. Such phenomena are termed entoptic. They may be of two lands : (1) extra-retinal, that is, due to opaque or semi- transparent bodies in any of the refractive structures anterior to the retina, and presenting the appearance of drops, strias, lines, twisted bodies, forms of grotesque shape, or minute black dots dancing before the eye ; and (2) intra-retinal, due to opacities, &c., in the layers of the retina, in front of Jacob s membrane. The iutra- retinal may be produced in a normal eye in various ways. (1) Throw a strong beam of light on the edge of the sclerotic, and a curious branched figure will be seen, which is an image of the retinal vessels. The construction of these images, usually called Purkinje s figures, will be understood from fig. 10. Thus, in the figure to the left, the rays passing Bl" A FIG. 10. Purkinje s Figures. In the eye to the right the illumination is through the sclerotic, and in the one to the left through the cornea. through the sclerotic at b," in the direction b"c, will throw a shadow of a vessel at c on the retina at b , and this will appear as a dark line at B. If the light move from // to a", the retinal shadow will move from b to a , and the line in the field of vision will pass from B to A. It may be shown that the distance c b corresponds to the distance of the retinal vessels from the layer of rods and cones (see ANATOMY, vol. i. p. 888), If the light enter the cornea, as in the figure to the right, and if the light be moved, the image will be displaced in the same direction as the light, if the movement does not extend beyond the middle of the cornea, but in the opposite direction to the light when the latter is moved up and down. Thus, if a be moved to a , d will be moved to d , the shadow on the retina from c
to c , and the image b to b . If, on the other hand, a be