ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
593 
a somewhat different way. Each facet acting as an astronomical tele- 
scope may he considered as imprinting an image on the retina. Thus 
for each point of the object there will be a “ summation image,” con- 
sisting of about thirty superposed images. 
On seeking to apply the principle of the image formation in the 
Lampyris-e ye to insect eyes in general, a difficulty arises owing to the 
fact that in many the space between the crystalline cones and the retina is 
for the most part filled with black pigment instead of with transparent 
material as in the Lampyris-e ye. Now it is known that the pigment of 
the Frog’s retina is by illumination driven forward between the rods so 
Fig. 71. 
as to separate the individual retinal elements, but that in the dark it 
passes back to the choroid, leaving the retina free. The author there- 
fore supposes that in the insect eye a similar jdienomenon takes place, 
and that the pigment regulates the brightness of the retinal image in a 
way analogous to that of the iris. This theory was put to the test in 
the case of Hydrophilus piceus , Dyticus marginalia , and Colymbetcs fuscm. 
