PHOTOMECHANICAL CHANGES IN THE RETINA 545 
Kuihne (’78) expressed the belief that the chemical products 
formed by the decomposition of retinal pigment through the 
‘action of light served to stimulate the visual cells sufficiently 
to cause the sensation of vision. Besides the visual purple 
and brown pigment (‘Fuscin’) further ‘Sehstoffe’ were postulated 
in order to explain such cases as albinism. When one reflects 
upon the minute amount of substance that can produce the 
sensation of smell, it appears very probable that cells as highly 
specialized as the rods and cones, if open to chemical stimula- 
tion of this kind, could find their adequate stimulus in some such 
way as Kihne suggested. The anatomical isolation of the rod 
and cone layer from blood vessels would also retard the removal 
of decomposition products and allow them to become effective. 
Essentially similar views have been advanced by Boll (’81) and 
Gad (94). At its best, however, this theory has many objections 
and it is doubtful if the retinal pigment acts in this way. 
A relation between the retinal pigment and the regeneration 
of visual purple was maintained by Ayres and Kihne (’78), 
who directly compared the pigment cell to a gland, the secretion 
of which is the visual purple. This conception was supported 
by experiments, among which the use of pilocarpin was found to 
reduce greatly the time of visual purple regeneration in the dog 
and rabbit; the similar effect of this drug upon the activity of 
glandular epithelium in general is well known. Dreser (’86) 
corroborated, on the frog, these results. Kiihne (’79) cites the 
parallelism between the time needed for the regeneration of visual 
purple in the frog and the length of time in which dark adaption 
is first accomplished, as evidence of a significant relationship ex- 
isting between the two. In mammals with their minute amount 
of retinal pigment, which undergoes but limited migration if any 
at all, these relations with the visual purple are not evident and 
a more inclusive explanation is demanded. This was furnished 
of the more recent examples is found in the elaborate theory of the significance of 
pigment migration advanced by Klein (’11), who assumes, on subjective grounds, 
the existence of extensive movements of the human retinal pigment. In a simi- 
lar way subjective observations have lead other writers to take for granted that 
the cone cells of man likewise undergo striking positional changes. 
