254 
MR. CLAUDET ON DIFFERENT PROPERTIES OF SOLAR RADIATION 
of the glass employed, would be an advantage, by dispersing, at the focus or on the 
plate, beyond the photogenic lines, the red, orange or yellow rays ; for the reason, 
that if they were brought to the same point they would tend to neutralize and destroy 
the effect of the photogenic rays. 
In October 1846, M. Lerebours announced to the Paris Academy of Sciences that 
the red rays prevented the action of the photogenic rays ; this announcement induced 
Messrs. Foucault and Fizeau to publish immediately similar results, which they had 
previously consigned to the Academy in a sealed memoir, bearing date May 1846. 
These communications of Messrs. Lerebours, Foucault and Fizeau, led Dr. Dra- 
per to write a letter, published in the Philosophical Magazine of February last, re 
peating his observations on the spectrum of Virginia, adding several other analogous 
facts confirming the theory of a protecting and even destroying action exercised by 
the least refrangible rays. Dr. Draper, in the same letter, said that the rays which 
protect the plate from ordinary photogenic action are themselves capable, when 
isolated, of producing a peculiar photogenic effect. 
Soon after the publication of M. Ed. Becquerel’s memoir, M. Gaudin made some 
analogous researches on the Daguerreotype plate ; and he succeeded in developing 
an image as perfect as that produced by mercury, by submitting the plate, when 
taken from the camera obscura, to the action of light alone under a yellow glass, and 
without any subsequent exposure to mercury. 
This curious discovery gave some hope that, from the supposed continuing action 
of the red and yellow glasses, by submitting the plate alternately, or simultaneously, 
to the action of the mercury and of these glasses, an accelerated development of the 
image would result ; but all the researches made to arrive at this point have been 
fruitless ; and, until the present time, the labours of Messrs. Becquerel and Gaudin 
have received no satisfactory explanation or useful application. 
My own experiments, which are the object of this memoir, seem to prove that 
M. Ed. Becquerel was mistaken as regards the Daguerreotype plate, in so far as 
he attributed to the red, orange and yellow glasses a continuing action of the effect 
of the photogenic rays. 
In the Daguerreotype, when we speak of the photogenic effect, we cannot under- 
stand any other than that which gives to the surface an affinity for mercurial vapour. 
In the case of photogenic papers, it is true that the red, orange, and yellow rays 
render the parts previously affected by the photogenic rays black or of a darker 
colour. It is the same with the Daguerreotype plate, which after it has been feebly 
impressed, darkens rapidly to a violet colour under the radiation of a red or yellow 
glass. This is the only continuing effect I have observed, and this effect is not con- 
tinuing in a Daguerreotype sense, it has no relation to the property of attracting the 
mercurial vapour ; on the contrary, it will be seen from the experiments which I am 
about to describe, that the radiations of red, orange and yellow glasses entirely de- 
stroy this property. There exists then a certain analogy between the action of the 
