VISION. 581 
into the facts of comparative anatomy and physiology, there 
are many of ‘a significance that we can not ignore; the impor- 
tance of light to most protoplasmic processes, such as the ac- 
cumulation of pigment in certain regions marking the very 
beginnings of eyes; the large amount of pigment found in the 
eyes of most groups of animals and of nearly all mammals sug- 
gesting that this is a provision for the retention of light, which 
we can scarcely conceive as acting in other than a chemical 
manner. At the same time, in keeping with the spirit of this 
work throughout, we suggest caution in believing that explana- 
tions based on our limited experience are the only ones possible. 
It is worth while to bear in mind, however, that currents of 
rest and currents of action similar to those demonstrated to 
exist in muscle, glands, nerves, etc., may be shown to exist in 
the retina. In all the other cases these are in intensity parallel 
to the degree of functional (and chemical) activity of the part, 
and it makes the probability of there being a chemistry of the 
retina as a foundation for the impulses therein generated great- 
er. The subject is as yet, however, in the region rather of 
speculation than of ascertained fact. 
The Laws of Retinal Stimulation,—It may be noticed that, when 
a circular saw in a mill is rotated with extreme rapidity, it seems 
to be at rest. 
If a stick on fire at one end be rapidly moved about, there 
seems to be a continuous fiery circle. , 
If a top painted in sections with various colors be spun, the 
different colors can not be distinguished, but there is a color 
resulting from the blending of the sensations from them all, 
which will be white if the spectral colors be employed. 
When, on a dark night, a moving animal is illuminated by 
a flash of lightning, it seems to be at rest, though the attitude 
is one we know to be appropriate for it during locomotion. 
It becomes necessary to explain these and similar phe- 
nomena. Another observation or two will furnish the data for 
the solution. 
If on awakening in the morning, when the eyes have been 
well rested and the retina is therefore not so readily fatigued, 
one looks at the window for a few seconds and then closes the 
eyes, he may perceive that the picture still remains visible as 
a positive after-image ; while, if a light be gazed upon at night 
and the eyes suddenly closed, an after-image of the light may 
be observed. 
It thus appears, then, that the impression or sensation out- 
