EYE AS A CAMERA OBSCURA. 343 
GD Sia Fe ae 
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condition in which that of a cat, or dog, or ox is, when subdued light causes 
the iris to expand, and allows the reflecting tapetum to come into play, so that 
the considerations which I have to urge apply to the mammal as much as to the 
fish, provided they are taken with pupils equally dilated ; but as the tapetum in 
the shark is very large, very brilliant, and always in action, I shall restrict myself 
for the present to it. 
The light, which penetrates to the bottom of a shark’s eye, will, in part, be re- 
flected from the retina (a phenomenon which for the present I disregard), in part 
traverse it, and reach the tapetum, where a portion will be lost by absorption and 
irregular reflection or dispersion, and (what alone concerns us here) in part un- 
dergo direct reflection, return through the retina, and escape by the pupil. This 
returned light will impress the retina in traversing it, and illuminate external 
. objects on leaving the eye. 
The first question, then, is, “‘ How will this light impress the retina?” Ac- 
cording to J. Mitrer and W. Macxenzix, as we have already seen, only inju- 
: riously, so far as freedom from the sensation of dazzling, or distinctness of 
I visual perception, are concerned ; according to Topp and Bowman “probably” by 
“increasing the visual power, particularly when the quantity of light admitted 
into the eye is small.”* I have urged elsewhere that ‘‘ what is equivalent to 
two rays of light falling upon the retina will produce two impressions. We send 
a capillary sunbeam through the retina in one direction, and instantly return it 
_ __ through that membrane, a little diminished in intensity, in the opposite direction ; 
if it determined a sensation in its first passage, what is there to prevent its doing 
__ soinitssecond? If, for simplicity’s sake, we suppose exactly the same points 
of the retina to be traversed by the incident and the reflected ray, then (unless 
the luminous intensity of the incident ray was so great as by its passage to ex- 
 haust the sensibility of the retina), the reflected ray will repeat somewhat less 
powerfully the impression made by the incident one. The difference will be as 
great as there is between a sound and its echo, but not greater. 
«On this view of matters, the tapetum, especially in twilight, will serve the 
important purpose of making every perceived ray of light tell twice upon the 
retina, so that the sensation it produces will either be increased in distinctness or 
_ in duration, and probably in both.” + 
. I willnot deny that we are not entitled at once to infer that because a molecular 
_ change (modulation, vibration, polarization?) transmitted through a special struc- 
ture in one direction produces a peculiar sensation, it will certainly produce the 
_ same sensation on being transmitted through that structure in the opposite direc- 
3 _ tion ; but there are strong analogies in favour of such a view, and it is entitled to 
_ be regarded as a likely hypothesis. 
* Physiology of Man, chap. xvii., p. 23. + Researches on Colour-Blindness, p. 99. 
VOL. XXI. PART. II. 42 
