480 M. A. Claudet on the Dynactinometer, an Imtrument 



means of ascertaining the time position of both in all circum- 

 stances ; so that photographers may now depend on a well-de- 

 fined image with any lenses, and with those which before were on 

 that account quite imperfect and useless. 



The new and unexpected phsenomenon of the variation of the 

 distance separating the two foci has been received with the same 

 doubts as M'as at first the fact of their non-coincidence ; because 

 the effect cannot be explained by any of the propei'ties which 

 are known to affect the refraction and dispersion of the various 

 rays of the spectrum, when they are refracted from the atmo- 

 sphere through object-glasses. 



We reason too much as if in photography there were nothing 

 more than the theory of the light which affects our vision, the 

 theory by which telescopes, and other instruments destined to 

 form a visual image, are generally constructed. But we must 

 abandon that theoiy, and study the new laws of photography, 

 which alone will afford us the means of producing a perfect pho- 

 tographic apparatus, and explaining a fact purely photogenic. 



By means of my focimeter, an instrument I have contrived, 

 and which every photographer can construct himself (see figs. 4 

 and 5, Plate III., and description in Phil. IVlag. for Nov. 1849), 

 only a few series of experiments are required to decide the question. 

 Let us hope that such an important fact will not remain long in 

 that state of doubt, which is not satisfactory either in a philoso- 

 phical or in a practical point of view. 



I shall now examine the question of the coincidence announced 

 by opticians to exist in their object-glasses. It is not correct to 

 say in a general way, that lenses can be constructed having their 

 two foci coincident ; it would be more so to say, that an optician 

 can produce lenses in which on an average, for a given distance, 

 but for that distance only, the two foci may be practically less 

 separated, or even may agree, sti'ictly speaking. In fact, thei"e are 

 no tolerably good lenses in which the two foci do not coincide for 

 one distance. But is it desirable to adopt cun^atures which render 

 the two foci coincident ? In doing so, is not the optician exposed 

 sometimes to sacrifice other important conditions, such as the 

 curvatures which correct the spherical aberration in the most 

 perfect manner ? And, moreover, in concentrating all the rays 

 to the same focus, does he not very often render his lenses slower 

 for the development of the photogenic action ? I shall leave the 

 first point for the investigation and decision of mathematicians, 

 and content myself with examining the question of the influence 

 ,of antagonistic rays, which may in certain circumstances, as I 

 shall hereafter explain, counteract the action of the photogenic 

 rays when they are all concentrated on the same points. 



It had always appeared to me that the lenses generally the 



