36 
G. H. Parker 
throTigh a single cone, became brigbter when the same kind of light 
feil on the surrounding cones. Repeated trials convinced me that 
this was not the case; at least, on sliding the diaphragm, I could 
perceive no change in the brightness of the rhabdome, though, when 
a piece of slightly smoked glass was introduced between the source 
of light and the microscope, the light in the rhabdome was per- 
ceptibly diminished. I therefore believe that the image received by 
the retina in Astacus when the pigment is arranged for bright 
illumination is an apposition image. 
The eharacter of the image in eyes in which the pigment is 
arranged for dim light was not easily determined on account of the 
difficulty of making preparations. This was necessarily done in the 
light, and, though the eyes were prepared immediately on being 
taken from the darkness, the exposure to light sufficed often for a 
complete return of the pigment to its position for bright illumination. 
I was, therefore, finally obliged to experiment with preparations cut, 
not at the level of the rhabdomes, but far enough distal to these 
to avoid all pigment. Taking as the body to be illuminated, the cut 
proximal end of a group of four cone-cells, I repeated the experiment 
which I had previously tried with the rhabdome, and found the group 
of cone-cells very much brighter when light was admitted to the 
retina through many cones than when it entered through only one, 
an Observation that indicated to my mind the presence of a super- 
position image. The image itself could be easily seen; thus, when 
a small lead-pencil was held in front of the prepared eye, its image 
appeared as a dark band in the retina, and I could readily demon- 
strate by moving it that the image was an erect one. The image 
was Sharp enough to enable me to distinguish the pointed from the 
blunt end pf the pencil, and, so far as I could see, the sharpness 
increased slightly on focusing proximally, so that the image may 
have been clearer at the level of the rhabdomes than where I ob- 
served it. These observations left no doubt in my mind that when the 
retinal pigment in Astacus was adjusted for very dim light, the 
image formed by the dioptric apparatus was a Superposition one. 
Hence Astacus may be classed, so far as its dioptric Organs are con- 
cerned, with those crustaceans whose retinal Images, as Exner has 
pointed out, are formed at one time as appositiou, at another as 
Superposition images. 
