428 Mr. It. Templeton on the received value of fi, 



14. The human lens, owing to its being less refractive, is far 

 the best to experiment on ; but none being procurable, many 

 experiments were made on ox-eyes to determine the position of 

 the focus of parallel rays. The eyes vary much in dimensions, 

 and therefore require the radii and distances to be carefully 

 measured in each case of trial; the position of the focus varies 

 also, in water from 40 to 48 millims. from the rear of the lens. 

 The best observation in a medium-sized eye gave 44 millims. 

 In air the focus would appear to be at 6 millims. ; but there is 

 great difficulty in measuring it; it is consequently doubtful. 

 While the lens was in the water, the diaphragm which transmitted 

 the parallel rays was shifted over different parts of the lens within 

 a small compass ; but the focus appeared to undergo no alteration 

 thereby. 



15. Assuming that the aqueous and vitreous humours have 

 in the ox-eye the same refractive indices as in the human eye, 

 calculation gives 1*5086* for the index of refraction of the ox- 

 eye lens, and the focus for parallel rays entering the cornea is 

 found to be 37*2 millims. from the back of the lens, or exactly 

 4 millims. more than the measured distance from the back of the 

 lens to the retina. The air-focus gives a somewhat higher re- 

 fractive index, supporting the opinion that the central parts of 

 the lens have a higher refractive power, since its focus was almost 

 exclusively of rays which passed direct through the centre, a 

 small diaphragm having been used. These results are not en- 

 titled to any particular confidence. 



16. From what has been stated above, it would appear to fol- 

 low that all that the retina requires for distinct vision is a cer- 

 tain amount of illumination on a certain-sized but minute disk 

 of its surface, — and, if so, that the eye is at all times prepared for 

 vision at every distance, the size of the pupil determining what 

 that distance shall be. To test this, a little frame was con- 

 structed to carry four diaphragms on the same imaginary axis 

 and parallel to each other: to the 



first a small piece of sheet-brass was 



affixed, in which, commanding the 



centres of the diaphragms, some 



small holes were pierced, all within 



a space not larger than the pupil, — 



the central hole just large enough 



for accurate vision^of distant objects; the others such as are in 



* " Dr.Wollaston has ascertained the refraction out of air into the centre 

 of the recent crystalline of oxen and sheep to be nearly as 143 to 100, into 

 the crystalline of fish and into the dried crystalline of sheep as 152 to 100." 

 — Dr. Young's " Lecture on the Mechanism of the Eve," Phil. Trans. 

 1801, p. 42. J 



