THE EYE. 



parency, and the crystalline humour is more especially liable to 

 this. In such cases vision is sometimes recovered by means of the 

 removal of the crystalline humour, in which case the organ is 

 reduced to two humours, the aqueous and the vitreous ; but as the 

 eye owes in a greater degree to the crystalline than to the other 

 humours the convergent power, it is necessary in this case to 

 supply the place of the crystalline by a very strong convergent 

 lens placed before the eye. 



32. 2. MAGNITUDE OF THE IMAGE ON" THE EETIXA. 



In order to obtain a perception of any visible object, it is not 

 enough that the image on the retina be distinct, it must also have 

 a certain magnitude. 



Let us suppose that a white circular disk, one foot diameter, 

 is placed before the eye at a distance of 57 feet. 



The axes of the pencils of rays proceeding from such disk to the 

 eye will be included within a cone, whose base is the disk, and 

 whose vertex is in the centre of the eye. . 



These axes, after intersecting at the centre of the eye, will form 

 another cone, whose base will be the image of the disk formed 

 upon the retina. The common angle of the two cones will in this 

 case be 1. 



Let A B (fig. 3), be the diameter of the disk. Let c be the 

 centre of the eye, and let b a be the diameter of the image on the 



retina. It is clear, from the perfect similarity of the triangles 

 A c B and a c b, that the diameter of the image b a will have to 

 the diameter of the object B A the same proportion as the distance 

 a c of the retina from the centre c has to the distance A c of the 

 object from the same centre. Therefore in this case, since one-half 

 the diameter of the eye is but half an inch, and the distance A c is 

 in this case supposed to be 57 feet, the magnitude of the diameter 

 b a of the image on the retina will be found by the following 

 proportion : 



a b : A B : : | : 57| X 12 = 690, 



Therefore we have 



X A B _ _6^ _ 1 



"" 690 ~ 



ab = 



690 



115 



The total magnitude, therefore, of the diameter of the image 01 

 60 



