- ‘ 
Marcu 14, 1884.] 
second surface, ground to fit it with accuracy 
when the mirror is in proper shape. If the 
mirror rested directly in contact with this second 
surface, no advantage would be gained, since 
the backing itself would bend as readily as the 
mirror. Therefore between the two is inserted 
a thin stratum of some elastic substance. Mr. 
Henry has found a fine sheet of flannel to give 
the best results. The effect of the sheet is 
to diminish the flexure of the mirror by a 
fraction depending on its stiffness and on the 
elasticity of the flannel. Theoretically it may 
be considered imperfect, because, in order to 
act, some stiffness is required in the mirror 
itself. A perfectly flexible mirror would bend 
just as much with the flannel as without it. 
But the flexure of the mirror can, it appears 
to me, be reduced to quite a small fraction of 
its amount. Moreover, I see no insuperable 
objection to the superposition of two systems 
of the kind; the mirror resting upon a stiff 
disk, which is itself supported upon a second 
one. This plan has been entirely successful in 
the cases in which it has been applied. Mir- 
rors up to twelve inches in length show not the 
slightest flexure when moved into all practical 
positions. Unfortunately it has not yet been 
tried with reflectors of a larger size. 
Simon NEWComs. 
AFTER-IMAGES. 
THAT one cannot well contribute to a subject un- 
less he knows something of its literature is illustrated 
afresh in a painstaking article by Mr. Sydney Hodges, 
in the October number of the Nineteenth century, 
on ‘ After-images.’? Mr. Hodges has discovered for 
himself the fact that the after-images of bright objects 
are in general colored, and that they change color as 
they gradually fade away in the dark field of vision 
when the eyes have been covered. He has very 
carefully observed the phenomena in his own case; 
and he comes to the conclusion, that, in all cases of 
such after-images, ‘‘the color of the image is pro- 
duced, not by the tint of the object we look at, but 
by the amount of light thrown on the retina, either 
by the greater or less intensity of light in the object 
itself, or by the amount of time during which one 
looks at it.” This remarkable result is, however, 
reached by experiments that cannot prove it: for in 
all of them the conditions are too complex; namely, 
in all the important cases, our experimenter observed 
the bright object for a comparatively long time before 
covering the eyes. The common theory of these 
phenomena, however, assumes, that, after such a 
continued observation, the causes of the colors in 
the after-image are decidedly complex; and their 
complexity may be such as to render a complete 
explanation of the phenomena wholly impossible. 
Therefore the only simple way to begin observing 
SCIENCE. 
321 
the phenomena is to get instantaneously produced 
after-images, and to observe the order of colors in 
them as they disappear: for the common theory is 
substantially, that the separate nervous elements, 
whatever they are, that respond to the different wave- 
lengths, or that produce, when excited, the three 
primary color-sensations, recover from the after- 
effects of excitement with different degrees of rapid- 
ity, and again, if continuously excited, yield to 
exhaustion with various degrees of speed; so that 
the color of the after-image at each instant, since it 
must depend on the mixture of the different after- 
effects in the different elements, must vary as these 
elements return, each at its own rate, to the condi- 
tion of rest, and must so depend, not only on the 
rates of recovery of each element, but also upon the 
degree of exhaustion that each element has under- 
gone during the time of stimulus. Hence the sim- 
plest case would be the one where the degree of 
previous excitement was as nearly as possible equal 
for the different elements, —a case which would be 
realized best through momentary stimulus. But if 
the stimulus is continued ten or twenty seconds, then 
the after-image will be further affected by the rates 
at which the different elements have tended to get 
exhausted; and if these rates are themselves quite 
different, as is likely, then the after-image will be 
determined in its successive colors, not only by 
the different rates of subsidence of excitement in the 
elements, but by the different degrees of previous 
exhaustion: and all this may possibly so complicate 
things as to make the phenomena of the after-image 
seem wholly out of relation to the color of the origi- 
nal object. And thus any such uniformity as our 
author notices will be of little worth, unless we know 
just the conditions of time and illumination, and 
unless we observe the results with very many persons; 
and even then the facts may turn out to be too com- 
plex for us to explain, so that no light will be thrown 
by them on the theory of after-images. 
All this Mr. Hodges could have found stated or 
implied in many places. The phenomena have been 
much observed and discussed. Helmholtz gives the 
older literature in § 23 of his Physiological optics, 
and himself declares that it is impossible, by reason 
of the complexity of the phenomena of fatigue, to 
give a complete explanation of these phases. Wundt, 
in the Physiologische psychologie, while not agree- 
ing as to the theory with Helmholtz, still holds to 
an explanation somewhat analogous; and he consid- 
ers, that, to avoid confusion, one must clearly sepa- 
rate the cases of instantaneous stimulation from the 
more complex ones, in which, as he implies, fatigue 
and other causes may affect the phenomena (Op. cit., 
bd. i., p. 488, of 2d ed.). But of such separation 
our author is ignorant, and confuses all the phenom- 
ena in one mass together; so that observations that 
might easily have been made really valuable for the 
theory cannot well be used in their present shape at 
all, and can only raise in the casual reader’s mind 
a false hope that a law has been found, when, in fact, 
as it is stated, the supposed law of our author is 
false, and is at once contradicted by the observation, 
