222 HENRY LAURENS AND S. R. DETWILER 
is due to the fact that the pigment, which has moved back into 
the body of the epithehal cells as an effect of darkness, requires 
some time to be brought out again into the prolongations of the 
cells to ensheath the individual visual elements. 
Bard ('19), in a highly theoretical paper, in many respects 
offering views widely divergent from those usually held concern- 
ing vision of form and of color, explains many things by the 
assumption that both pigment migration and cone contraction 
take place in the human eye. 
Cobb ('19, p. 444) also states that pigment migration takes 
place in the human retina. He says : 
aside from the changes in the size of the pupil there are two anatomical 
factors undoubtedly concerned in dark and bright adaptation: the 
exhaustion and regeneration of visual purple (or possibly other photo- 
chemical substance) ; and the migration of the pigment of the hexagonal 
cells. This last may be a protecting device that acts fairly promptly, 
and has the effect of enclosing the retinal rods, and by its own light- 
absorbing qualities reducing the amount of light absorl^ed by the indi- 
vidual rods. It is conceivable that a sudden flash of light might antici- 
pate this action and produce a strong destruction of the photochemical 
material in a short time, before the pigment cells have had tune to 
react, while with gradual onset of light the time is adequate for the 
pigment cells effectively to assume this protective function. 
And on page 445 : 
Some of the curves strongly suggest two factors playing a part in 
dark adaptation .... allowing the interpretation that the 
results are arising from two more or less independent mechanisms one 
of which overtakes the other in effect, at the end of about four minutes. 
We believe that in the migration of pigment, the contraction 
of the cones, and the elongation of the rods there is exemplified 
the response of irritable protoplasm to a definite, adequate 
stimulus. In some cases the response is very marked, though 
of varying degree (fish, frog, bird) ; in others it is not demon- 
strable (man and mammals). In some it may serve an easily 
comprehended purpose; in others, in terms of the theory explain- 
ing it, it may seem to be useless. Nevertheless, if it can be 
demonstrated (as in the turtle, lizard, and alhgator) it cannot 
be explained away or ignored because it seems to serve no useful 
purpose. 
