fm- inwasunng the Intensity of the Photogenic Rays. 481 



most active were those in which the two foci were the most sepa- 

 rated. Such a result seems obvious when it is con'oborated by 

 the following facts and observations : — 



Sir John Herschel, from the commencement of the discoveiy 

 of photography, had expressed the opinion, that when lenses 

 were not achromatic, they had a greater photogenic power than 

 when achromatic, for the reason that in the former the photo- 

 genic rays were separated from the red, orange and yellow rays^ 

 endowed with an antagonistic action. 



M. Lerebours of Paris has proved, that, in throwing red, 

 orange or yellow rays on the photogenic part of the spectrum 

 fonned on a Daguerreotype plate, the action was entirely im- 

 peded or less rapid than when these last were insulated. 



I have myself demonstrated, by a long series of experiments 

 published in former memoirs, the antagonism of the red, orange 

 and yellow rays ; and I must not omit to mention, that Dr. 

 Draper of New York, and Messrs. Fizeau and Foucault of Paris, 

 have proved the same fact by some veiy conclusive experiments. 

 All these -observations tended to the conclusion, that the most 

 active lenses were those in which the photogenic focus was the 

 most separated from the visual focus; but we had no demon- 

 strable proof that the effect was a consequence of such a theory. 

 This has induced me to endeavour to find some means by which 

 the power of lenses having their foci separated could be com- 

 pared with those in which they coincide ; and I have contrived 

 an instrament which appears completely to fulfill this purpose, 

 and several others not less useful. 



I shall now proceed to describe the instiTiment which I call a 

 Dynactinometer (see fig. 3), its object being generally to measure 

 the actinic power resulting both from the intensity of the luminous 

 radiation and from the construction of lenses. It consists of a 

 thin metallic disc, perfectly black, having a slit extending fi'om 

 its centre to the circumference, fixed on an axis revolving through 

 a fixed metallic disc, perfectly white. The white disc has also a 

 slit from its centre of the exact length of the radius of the black 

 disc ; and by means of these two slits, and of their spiral sm-- 

 faces, the black disc can intersect the white disc, and by re- 

 volving, gradually cover the whole white ai'ea. The space of the 

 white surface on which the black disc can be superposed forms 

 itself a sort of dial, which is divided into any inmiber of equal 

 segments, all numbered. I have adopted the number of twenty 

 segments for a lai'ge circle inscribed on the dial, and of eight 

 segments for a smaller circle inscribed in the first. The first 

 twenty segments are numbered in simple arithmetical progression, 

 and the eight segments in geometrical progression, 1, 2, 4, 8, 

 16, 32, 64, The object of these two kmds of progression will 



