24 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
of blackness under the subsequent influence of a blue, violet, or lavender ray, as it 
would have done had no previous exposure to the red rays taken place. As far as 
my experiments go, also, it appears that every part of the spectrum impressed by the 
more refrangible rays, is equally reddened, or nearly so, by the subsequent action of 
the less refrangible. 
66. On the other hand, as regards the question, whether or not the simultaneous 
action of two rays of different refrangibilities be capable of producing photographic 
effects which neither acting separately is competent to produce, the following experi- 
ment will afford a satisfactory affirmative answer. A very fine and concentrated 
spectrum was formed, by receiving on an achromatic lens (by Cauche of Paris) of 
three inches aperture and twenty-six inches focal length, a sunbeam previously sepa- 
rated by the Fraunhofer flint prism above mentioned (Art. 5 7-)- The screen on 
which this spectrum was received having been carefully adjusted to the correct focus, 
a thin flint prism of small refracting angle was interposed between the lens and its 
focus, so as to intercept all the less refrangible rays from the green to the extreme 
red, and to throw them upwards by a second refraction, thereby superposing the image 
of the less refracted portion of the spectrum on that of the more refracted formed by 
a single refraction. And by varying the place of the second prism in respect of its 
distance from the screen, this superposition might be made to take place at any given 
point of the more refracted spectrum. Things being thus arranged, a slip of muriated 
paper was fixed on the screen so as to receive the simultaneous action of the rays in 
question. The results were very striking and beautiful. The blackening power of 
the more refrangible rays seemed to be suspended over all that portion on which the 
less refrangible fell, and the shades of green and sombre blue which the latter would 
have impressed on a white paper, were produced on that portion which, but for their 
action, would have been merely blackened. But in place of the dull brick-red fringe* 
which would have bounded this part of the spectrum on a white ground, and in place 
of the white prolongation (Art. 60 .) which would in that case have marked the re- 
gion of the mean and extreme red, there was produced a full, rich and fiery red, oc- 
cupying the whole of that region, and passing into a sombre crimson or purple, where 
the action of the extreme red rays ceased to counterbalance or modify that of the 
more refrangible ones, in concert with which they acted. The general effect was 
not unlike that of the succession of tints in the second series of the Newtonian rings, 
or rather, of the transition from the third into the second, only with a less vivid de- 
velopment of every colour but the red. 
67 . To those who may be disposed to repeat this and the following experiments, 
I would here observe that a polar axis driven by clockwork, to follow the di- 
urnal movement of the sun and fix the spectrum, is indispensable. But, as the last 
niceties of astronomical observation have as yet no place in this research, the inter- 
rupted motion of a pendulum clock, which is fatal in such observations, is not here 
* See Table in Art. 54. 
