26 
G. H. Parker 
through the rhabdome again by tbese cells, tbus doubtless increasing 
tbe Stimulation of tbat organ. Tbe fact tbat reflection would be 
most needed in faint ligbt and tbat under such circumstances tbe 
accessory cells are in a position to act most efficiently as reflectors 
suggests tbis as tbeir function. 
Tbe blackisb pigment grains are contained witbin tbe distal and 
proximal retinular cells, and in tbeir migrations tbey remain entirely 
witbin tbe walls of tbese cells. Since tbe cells bave about tbe 
same form after tbe migration of tbe pigment as before, I find it 
difficult to explain tbis cbange witbout assuming tbat it is produced 
as in vertebrates by a protoplasmic Streaming. Certainly in Astacus 
no muscles are concerned in tbe cbange, tbougb tbe case may be 
different in Palaemon. 
By way of summary it may be stated tbat tbe black pigment 
of tbe distal and proximal retinular cells is a means of Controlling 
tbe amount and quality of tbe ligbt tbat reacbes tbe rbabdomes. 
In comparative darkness, the absence of pigment around tbe rbab- 
dome, as well as tbe absence or shortness of the pigmented pro- 
cesses from the distal retinular cells, renders the rhabdome far more 
easily accessible to ligbt tban in tbe conditions presented in tbe 
brigbtly illuminated eye. In faint light, rays from a variety of 
directions can reach tbe rhabdome ; in brigbt ligbt, the more oblique 
rays are excluded and only tbose more nearly parallel witb the 
ommatidial axis enter it. When least ligbt enters tbe retina, tbe 
accessory pigment cells are most exposed and can consequently act 
most effectively as a reflecting apparatus. 
d. Theories of vision. 
Tbe numerous theories tbat bave been proposed to account for 
vision in compound eyes differ from one anotber in one or botb of 
tbe following particulars: first, as to which structure in tbe eye is 
tbe perceptive organ, i. e., is concerned witb the reception of ligbt 
and tbe production of tbe impulse transmitted by tbe optic nerve; 
and, secondly, as to the number of simultaneous but distinct im- 
pressions tbat eacb such organ can receive, i. e., wbetber each organ 
is a perceptive surface for a wbole picture or for only one element 
in a picture. 
In determining wbich are the perceptive Organs, facts concerning 
tbe structure of tbe eye are of no small importance; tbese organs 
