FURTHER REMARKS ON THE COMPOUND EYE. 575 



compound eye has been rendered most improbable by a more 

 complete knowledge of the structure of the compound eye, and 

 the above optical considerations are strongly in favour of the 

 more recent view, that the simple and compound eyes have 

 been perfected by divergent evolutionary stages. 



b. On the Illumination of the Retinal End Organs under the Dioptric 



and Mosaic Theories of Vision. 



Allusion has already been made (p. 556) to the unsatisfac- 

 tory character of the ' mosaic theory ' of vision in relation to 

 the illumination of the retinal image, and it has been stated 

 that the corneal lenses cannot increase the quantity of light 

 which acts on the retina, unless a real image is formed upon it 

 by a second refraction. It is perhaps necessary to elucidate 

 this statement more fully. 



It will be seen that a single lens cannot intensify the illumina- 

 tion of the retinal end organs of an ommateum, by the 

 following (PI. XL., Fig. 3): 



Suppose a plane luminous wave of the intensity / to fall upon 

 the surface I l = a, and to be transmitted to /' I' without appre- 

 ciable loss of intensity, the illumination of the surface /' I' will 

 be a I. 



Next let the surface / / be a convex refractive surface which 

 condenses the wave, and brings it to a focus /at a distance d 

 from //; at the distance 2d from 1 1 the wave will have the same 

 intensity as at / /, or again a I. Let the receptive surface /' /' 

 be at a distance n d from/, then the illumination of I' I' will be 

 only l> ; since it is evident that all the light which falls upon 



the segments of the lens I p I p will afterwards diverge to the 

 segment n q n q, and the efficient wave front which acts on n n 

 will be the same as that which falls on / /, although the image 

 at r r is more intensely illuminated than if the segments I p I p 

 were protected by an opaque diaphragm. 



If, however, a second refraction occurs, beyond r y, which 

 brings the rays of light in the segments nq nq to a focus with 

 those which fall on n n, the illumination of I' I' will be in- 



38 



