28 
G. H. Parker 
braiü, and that no other structure in tlie retina possesses both of 
these peculiarities , is strong evidence in favor of the perceptive 
nature of this body. Moreover, granting this concliisionj the changes 
undergone by the pigment become perfectly intelligible, for they are 
obvionsly directed toward Controlling the quantity and the quality 
of light that reaches the rhabdome. These various reasons, I be- 
lieve, justify the conclusion that the rhabdome is the perceptive 
Organ of the retina. 
Having reached this point, there still remains the question con- 
cerning the character of the impressions made upon the rhabdomes: 
does each rhabdome perceive a complete picture or only one de- 
ment in a picture? This problem may be approached from tvro 
sides; we may first try to ascertain the character of the imag^e 
thrown on the rhabdome by the dioptric apparatus, and secondly, by 
examining the finer structure of the rhabdome itself, we may 
attempt to determine to what extent it is capable of perceiving an 
image. 
Concerning the character of the image thrown by the dioptric 
apparatus, there are already a number of conflicting observations and 
opinions. From the time of Leeuwenhoek, investigators have been more 
or less familiär with the fact that each facet in the corneal cuticula is 
capable of projecting at a short distance behind itself a small in- 
verted image of an object held in front of it. In a carefully pre- 
pared fly's eye, Gottsche (52, pag. 488) believed that this image 
was to be seen at the proximal end of the cone, and, regarding it 
as functionally significant, he raised the question of how and where 
it is perceived. Max Schultze (68, pag. 27), who, as we have 
already seen, correctly interpreted the rhabdome as the perceptive 
organ of the retina, took the Suggestion from Zenker that the den- 
ser material of the proximal part of the cone-cells might cause a 
proximal displacement of one of the foci of the dioptric apparatus, 
and believed that in this way an image might be thrown on the 
rhabdome. Viallanes (92, pag. 372) states that, in the eyes of 
insects and crustaceans prepared by a method essentially similar to 
Gottsche's, a small inverted image could be observed at the distal 
end of each retinula. It is, then, a matter of Observation that for 
each ommatidium in freshly prepared eyes a small inverted image 
occurs not far from the distal end of the rhabdome. 
These observations are directly opposed to those of Exner (91), 
who, after preparing the rather resistant dioptric apparatus of Lam- 
