on the Mechanism of the Eye. 
%5 
EXPLANATION OF THE FIGURES. 
Plate II. Fig. 1. See Page 28. Prop. III. 
Fig. 2. See Page 28. Prop. IV. 
Fig. 3. See Page 31. Prop. V. 
Fig. 4 — 6 . Relating to the optometer. See Page 34. 
Plate III. Fig. 7. The form of the ends of the optometer, 
when made of card. The apertures in the shoulders are for 
holding a lens : the square ends turn under, and are fastened 
together. 
Fig. 8. The scale of the optometer. The middle line is 
divided, from the lower end, into inches. The next column 
shows the number of a concave lens requisite for a short- 
sighted eye ; by looking through the slider and observing the 
number opposite to which the intersection appears when most 
remote. By observing the place of apparent intersection when 
nearest, the number requisite will be found in the other column, 
provided that the eye have the average power of accommoda- 
tion. At the other end, the middle line is graduated for ex- 
tending the scale of inches by means of a lens four inches in 
focus ; the negative numbers implying that such rays as pro- 
ceed from them are made to converge towards a point on the 
other side of the lens. The other column shows the focal length 
of convex glasses required by those eyes to which the inter- 
section appears, when nearest, opposite the respective places of 
the numbers. 
Fig. 9. A side view of the optometer, half its size. 
Fig. 10. The appearance of the lines through the slider. 
Fig. 11. Method of measuring the magnitude of an image 
on the retina. See Page 48. 
