The Eetina and Optic Ganglia in Decapods, especially in Astacus. 29 
'pyrus by carefully removing tbe nervous layer of the retina and all 
pigment, observed, near the level formerly occupied by the percep- 
tive layer of the retina, a single erect Image produced by the com- 
bined action of the separate dioptric Organs of the eye. These 
Organs when uninjured never gave rise to small inverted Images as 
described by other investigators. The contrast between Exner’s 
observations and those summarised in the preceding paragraph 
must be evident. 
The grounds upon which these two opinions are based are not 
wholly unassailable ; in the method of preparation used by Gottsche 
and ViALLANES, the uncertainty as to vrhether the structures left in the 
eye retain their normal positions or not must be evident to any one 
who has attempted such experiments, and the complete removal of 
pigment, as practised by Exner, obviously leaves the dioptric ap- 
paratus in a condition far from normal. It is also important to bear 
in mind that neither Exner nor the advocates of the opposing theory 
have ever observed images in or on the perceptive organs them- 
selves; what they have seen has been located near the perceptive 
elements or at best in a position formerly occupied by these bodies. 
That the character of the image in the rhabdome itself has not been 
ascertained is due in great part to the technical difficulties involved 
in such an Operation. Astacus^ however, on account of the yery 
large size of its ommatidia, is more favorable for such experiments 
than other crustaceans; and it is chiefly due to this fact, rather 
than to any technical advance, that I have been able to observe the 
image in the rhabdomes of this crustacean. 
For this purpose I prepared the eyes in the following manner. 
A perfectly fresh eyestalk was so placed in a freezing microtome 
that, beginning at its proximal end, transverse sections could be cut 
from it tili only the hemispherical distal end containing the retina 
remained. It was easy to determine from the appearance of the 
pigment on the cut surface what plane in the retina had been 
reached, and there was no difficulty in making preparations for any 
desired level. The preparation having been cut, it was removed 
from the ice and placed on a cover-glass, its flat face being next 
the glass. This transfer was made before the retina had thawed out; 
for, after it had become soft, it was found impossible to move it 
without seriously displacing the ommatidia. The preparation was 
usually fixed to the cover-glass by a very small amount of Canada 
balsam, care being taken, however, to keep the balsam from entering 
