.588 



ON THE MECHANISM OF THE EYE. 



I next fixed the cancellated micrometer 

 at a proper distance, illuminated it strongly, 

 and viewed it through a pin hole, by which 

 means it became distinct in every state of the 

 eye ; and, looking with the other eye into a 

 small glass, I compared the image with the 

 micrometer, in the manner already described. 

 I then changed the focal distance of the eye, 

 so that the lucid points appeared to spread 

 into surfaces, from being too remote for per- 

 fect vision ; and I noted, 6n the scale, the 

 distance of their centres ; but that distance 

 jvas invariable. 



Lastly, I drew a diagonal scale, with a 

 diamond, on a looking glass, (Plate 9. Fig. 

 76.) and brought the images into contact 

 with the lines of the scale. Tlien, since the 

 image of the eye occupies, on the surface of 

 a glass, half its real dimensions, at whatever 

 distance it is viewed, its true size is always 

 double the measure thus obtained. I illumi- 

 nated the glass strongly, and made a perfo- 

 ration in a narrow slip of black card, which 

 I held between the images ; and was thus 

 enabled to compare them with the scale, al- 

 though their apparent distance was double 

 that of the scale. I viewed them in all states 

 of the eye ; but I could perceive no variation 

 in the interval between them. 



The sufficiency of these methods may be 

 thus demonstrated. Make a pressure along 

 the edge of the upper eyelid with any small 

 cylinder, for instance a pencil, and the op- 

 tometer will show that ihc focus of horizontal 

 rays is a little elongated, while that of verti- 

 cal rays is shortened ; an eifeet which can 

 only be owing to a change of curvature in 

 the cornea. Not only the apparatus here 

 described, but even the eye unassisted, will 

 be capable of discovering a considerable 



change in the images reflected from the cor- 

 nea, although the change be much smaller 

 than that which is requisite for the accom- 

 modation of the eye to different distances. 

 On the whole, I cannot hesitate to conclude, 

 that if the radius of the cornea were dimi- 

 nished but one tvvencieth, the change would 

 be very readily perceptible by some of the 

 experiments related ; and the whole altera- 

 tion of the eye requires one fifth. 



But a much more accurate and decisive 

 experiment remains. I take, out of a small 

 botanical microscope, a double convex lens, 

 of eight tenths radius and focal distance, 

 fixed in a socket one fifth of an inch in 

 depth ; securing its edges with wax, I drop 

 into the socket a little water, nearly cold, 

 till three fourths full, and then apply it to my 

 eye, so that the cornea enters half way 

 into it, and is every where in contact with 

 the water. (Plate 9. Fig. 77). My eye 

 immediately becomes presbyopic, and the 

 refractive power of the lens, which is re- 

 duced by the water to a focal length of 

 al)oul 16 tenths, is not sufficient to sup- 

 ply the place of the cornea, rendered in- 

 efficacious by the intervention of the water ; 

 but the addition of another lens, of five 

 inches and a half focus, restores my eye to 

 its natural state, and somewhat more. I 

 then apply the optometer, and I find the 

 same inequality in the horizontal and verti- 

 cal refractions as without the water ; and I 

 have, in both directions, a power of accom- 

 modation equivalent to a focal length of four 

 inches, as before. At first sight indeed, the 

 accommodation appears to be somewhat less, 

 and only able to bring the eye from the 

 state fitted for parallel rays to a focus at five 

 inches distance; and this made me once 



