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 (Pl. XL., Fig. 3): 
Suppose a plane luminous wave of the intensity J to fall upon 
the surface / /=a, and to be transmitted to /’ /’ without appre- 
ciable loss of intensity, the illumination of the surface /' I’ will 
bea I. 
Next let the surface / 1 be a convex refractive surface which 
condenses the wave, and brings it to a focus fat a distance d 
from 11; at the distance 2d from /1 the wave will have the same 
intensity as at //, or againaJ. Let the receptive surface I’ /’ 
be at a distance n d from f, then the illumination of II’ will be 
only a since it is evident that all the light which falls upon 
the segments of the lens // /p will afterwards diverge to the 
segment  q mq, and the efficient wave front which acts on n » 
will be the same as that which falls on //, although the image 
at y y is more intensely illuminated than if the segments / 1p 
were protected by an opaque diaphragm. 
If, however, a second refraction occurs, beyond y ry, which 
brings the rays of light in the segments nq nq to a focus with 
those which fall on mn», the illumination of J’ I’ will be in- 
38 
