580 ANIMAL PHYSIOLOGY. 
matical calculations, based on data derived from the experi- 
ment just described, locate the part concerned in the layer of 
rods and cones. Putting together all the facts of experiment 
with those derived from pathological conditions, there seems 
to be no reasonable doubt that the rods and cones of the Tetina 
are the seat of origin of the visual impulses. 
The performance of the experiment as given above requires 
usually two persons, but there are simpler methods, which, in 
some cases, also bring out the figures more satisfactorily: (1) 
It often suffices to move the head back and forward before the 
tube of a microscope without its objective; or (2) to move rapidly 
a card with a pin-hole held close before the eye, while the sub- 
ject gazes at a bright clear sky. When the card is moved from 
side to side, the vertical vessels are seen; if up and down, the 
horizontal. The shadows of the capillaries come out especially 
well by this method. It is essential, however, whatever plan 
be adopted, to gaze into infinite distance, as it were, in order 
fully to relax the accommodation and to avoid excessive ex- 
pectancy, which frustrates the former attempts at relaxation. 
The Nature of the Processes which originate Visual Impulses.— 
Much interest attached at one time to visual purple (rhodop- 
sin), because it was hoped that it might furnish a chemical ex- 
planation of vision. It was found that in certain animals, as 
frogs, when kept in darkness, the visual purple was renewed 
after having been bleached out by exposure to light; indeed an 
exact “ optogram,” or picture of an object, might be made and 
by appropriate reagents fixed on the retina as a bleached part 
of the visual purple. 
This substance is found exclusively in the outer limbs of the 
rods and not at all in the cones; but, since the retinas of some 
animals (snakes) are destitute of rods, and visual purple is also 
wanting in the macula lutea and fovea centralis of man and 
the apes, the points of greatest retinal sensibility, it is manifest 
that the theory based upon its presence breaks down as an ex- 
planation of vision, if to be applied universally; besides, the 
retinas of some animals with rods (dove, hen, bat) are entirely 
devoid of. visual purple. 
But, though this particular method of application of chem- 
istry to the explanation of the origin of retinal impulses has 
failed, it does not follow that a chemical theory as such is false, 
though it must be admitted that the evidence is as yet very 
incomplete on which to found such an explanation. 
But when we consider the evolution of the eye, and examine 
