the Index of Refraction of the Lens. 



427 



retina ; the focus will be further and further removed the nearer 

 the object is approached to the eye. 



10. When the human lens has been removed by the operation 

 of couching, the person operated on has sight so far restored that 

 he can go about*, distinguish clearly light from shade, and 

 be conscious of the presence of obstacles. We can realize the 

 condition of such persons by placing before the eye a glass lens 

 of 50 to 100 millims. negative focus ; by, however, interposing 

 a small hole (Nos. 2, 3, or 4 of preceding Table) in a sheet of 

 brass between the glass and the eye, vision is perfectly restored, 

 though for obvious reasons the objects regarded appear faint. 

 In these circumstances the focus of rays from a distant object is 

 far beyond the retina. 



11. Tracing into the eye an exterior ray of one of these coni- 

 cal pencils (that is, one making an angle of 9' 34" with the axis 

 of the eye), we find it come to a focus at a distance behind 22*2 

 millims., the distance from the cornea we have assumed for the 

 focus of parallel rays — that is, for the retina. Under/'" in the 

 annexed Table are given these focal distances for each of the 

 experimental holes. 



Nos. 



/'". 



e. 



0- 



e tan <p. 





millims. 



millims. 





millim. 



1 



42-94 



20-74 



6 46 



•041 



2 



31-36 



916 



15 22-7 



•041 



3 



28-80 



6-60 



21 24 



•041 



4 



25-65 



3-45 



41 11-8 



•041 



5 



24-45 



2-25 



63 331 



•042 



6 



24-00 



1-80 



79 50-5 



•042 



7 



23-88 



1-68 



85 51-2 



•042 



8 



23-70 



117 



124 22 6 



•042 



12. Subtracting 22*2 millims. from these several values of/'", 

 we have the quantities in the adjoining column under e. These 

 are the distances at which the foci are respectively formed in 

 rear of the retina. In the fourth column we have the angle 

 which the exterior refracted ray of each pencil makes with 

 the axis. 



13. Taking the first, or any of these values of e, and multi- 

 plying it by the tangent of the angle ($) corresponding to it, 

 we have approximately the radius of the minute disk formed on 

 the retina by the collective light of each pencil. Curiously 

 enough, this value ('041 millim.) is much about the same quan- 

 tity given above for the depth of the perceptive layer of the retina. 



* Vide Philosophical Transactions, 1801, p. 66. 



