26 
G. H. Parker 
througli the rhabdome again by these cells, thus doubtless increasing 
the Stimulation of that organ. The fact that reflection would be 
most needed in faint light and tbat under such circumstances the 
accessory cells are in a position to act most efficiently as reflectors 
suggests this as their function. 
The blackish pigment grains are contained within the distal and 
proximal retinular cells, and in their migrations they remain entirely 
within the walls of these cells. Since the cells bave about the 
same form after the migration of the pigment as before, I find it 
difficult to explain this change without assuming that it is produced 
as in vertebrates by a protoplasmic streaming. Certainly in Astacus 
no muscles are concerned in the change, though the case may be 
different in Palaemon. 
By way of summary it may be stated that the black pigment 
of the distal and proximal retinular cells is a means of Controlling 
the amount and quality of the light that reaclies the rhabdomes. 
In comparative darkness, the absence of pigment around the rhab- 
dome, as well as the absence or shortness of the pigmented pro- 
cesses from the distal retinular cells, renders the rhabdome far more 
easily accessible to light than in the conditions presented in the 
brightly illuminated eye. In faint light, rays from a variety of 
directions can reach the rhabdome ; in bright light, the more oblique 
rays are excluded and only those more nearly parallel with the 
ommatidial axis enter it. When least light enters the retina, the 
accessory pigment cells are most exposed and can consequently act 
most effectively as a reflecting apparatus. 
d. Theories of vision. 
The numerous theories that bave been proposed to account for 
vision in compound eyes differ from one another in one or both of 
the follo wing particulars: first, as to which structure in the eye is 
the perceptive organ, i. e., is concerned with the reception of light 
and the production of the impulse transmitted by the optic nerve; 
and, secondly, as to the number of simultaneous but distinct im- 
pressions that each such organ can receive, i. e., whether each organ 
is a perceptive surface for a whole picture or for only one dement 
in a picture. 
In determining which are the perceptive organs, facts concerning 
the structure of the eye are of no small iraportance; these organs 
