604 ON THE MECHANISM OF THE EVf.. 



EXPLANATION OF THE FIGURES. 



Plate 9. Fig. 71. 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. 72. The scale of the optometer. The middle line is divided, from the lower end, into 

 inches. The right hand 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 in- 

 tersection appears when most remote. 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 proceed 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, re- 

 quired by those eyes, to which the intersection appears, when nearest, opposite to the respec- 

 tive places of their numbers. 



Fig. 73. A side view of the optometer, half its size. 



Fig. 74. The appearance of the lines through the slider. 



Fig. 75. Method of measuring the magnitude of an image on the retina. 



Fig. 76. Diagonal scale drawn on a looking glass. 



Fig. 77. The method of applying a lens with water to the cornea. 



Fig. 78. The appearance of a spectrum occasioned by pressure; and the inflection of 

 straight lines seen within the limits of the spectrum. 



Fig. 79- -An illustration of the enlargement of the image, which would be the conse- 

 quence of an elongation of the eye: the images of the candles, which, in one instance, fall 

 on the insertion of the nerve, falling, in the other instance, beyond it. 



Plate 10. Fig. 80. The successive forms of the image of a large distant object, as it 

 would be delilieated by each refractive surface in the eye ; to show how that form at last 

 coincides with the retina. £ G is the distance between the foci of horizontal and vertical 

 rays in my eye. 



Plate H. Fig. 81. Vertical section of my right eye, seen from without; twice the 

 natural size. 



Fig. 82. Horizontal section, seen from above. 



Fig. 83. Front view of my left eye, when the pupil is contracted ; of the natural size. 



Fig. 84. 'J he same view when the pupil is dilated. 



Fig. 8.5. Outline of the eye and its straight muscles when at rest. 



