536 LESLIE B. AREY 
with an account of certain theoretical considerations regarding 
the significance of these discoveries. No attempt will be made 
to trace the complete history of the investigations demonstrating 
the effect of light on the retinal elements, but only to outline 
the principal contributions that are of historical interest, as 
well as those that show the present status of the subject. In 
doing this, however, there will be included certain results that 
have escaped the attention of previous reviewers. 
The rods, cones, and pigment in the retinas of many verte- 
brates undergo positional changes when adequately stimulated. 
A considerable number of investigators (since the first exact work 
by Mann ’94) have also shown that changes occur in the bi- 
polar and ganglionic cells of the retina through the influence of 
light. These workers, though often contradicting one another, 
have described changes in the shape and size of the retinal 
nuclei, and especially in the Nissl granules of the ganglionic 
cells, as due to the action of light. The occurrence of definite 
positional changes in these elements, however, has not been 
. satisfactorily established. 
For the sake of clearness it is preferable to trace separately 
the three lines of discovery which have demonstrated movements 
in the retinal pigment, in the cones, and in the rods. [t should 
be remembered, however, that these three divisions are not 
historically isolated but are intimately connected through the 
overlapping results of many observers. 
A variability in the degree to which the retinal pigment ex- 
tended had been noted several years before the cause was sus- 
pected. Thus, H. Miiller (56) not only observed this varia- 
bility but a'so saw that the pigment in many cases accumulated 
near the tips of the cones. Morano (’72) likewise questioned 
the constancy of the pigment distribution, stating that in some 
retinas the pigment extended only half the length of the outer 
member of the rods. 
The first observation concerning the effect of light upon the 
retinal pigment was recorded by Czerny (’67), who, after con- 
centrating sunlight on the retinas of various animals, found 
that the pigmented epithelium and the retina were less easily 
