for measuring the Intensity of the Photogenic Rays. 489 



For the telescope, the question is, knowing the density and 

 dispersion of two glasses of different indices of refraction, to 

 combine them so that all the rays producing white light shall 

 converge to the same point. But the same two glasses have a 

 dispersion more considerable for the photogenic rays than for 

 any others ; the indices of refraction of these photogenic rays ai-e 

 much greater than any rays forming white light. Therefore, in 

 the construction of object-glasses, opticians must determine the 

 curvatures of the two glasses according to the refraction and 

 dispersion only of the photogenic rays, without regard to the 

 refraction and dispersion of the luminous rays. Lenses made by 

 this plan will probably be subject to a considerable chromatic 

 aberration for the visual rays ; they may not give a veiy well- 

 defined image on the ground-glass, but certainly they will pro- 

 duce the most perfect pictures on the photographic surface ; 

 and this is the only important point for a photographer. 



Before I conclude, I shall call the attention of physicists to 

 the influence which may be exercised on the achromatism of 

 photographic object-glasses by the polarization of light. 



When we consider that light, which is always more or less 

 polarized according to the state of the atmosphere, is first reflected 

 at various angles on the fii-st surface of the lens, and fi'om thence 

 is to be refracted through four different thicknesses of glasses 

 more or less tempered in all their parts, that this refraction takes 

 place at different angles for every point of the surface, it may 

 be allowed to suppose, that, according to the common laws of 

 polarization, certain rays will be extinguished, sometimes for the 

 points distant from the centre, which must modify the colour of 

 the light given by these various points, and consequently exer- 

 cise a certain influence on their achromatism. Then the two 

 foci may be more or less separated, for the same reasons which I 

 have given before. 



Not having yet been able to experiment on these facts suffi- 

 ciently, I now offer only a suggestion which appears to me to 

 deserve serious investigation, and which must be constantly in 

 the mind of the observer, as well as all the other phsenomena 

 referring to light. I have been induced to believe that the polari- 

 zation of light exercises a certain influence on the achromatism 

 of object-glasses, from the fact that when I use a prism or mirror 

 for reinverting the image, it often happens that the two foci are 

 more or less separated than when I operate with the same light 

 without reflecting the image of the object. 



I must add another observation. Several photographers, par- 

 ticularly amateurs, who generally take views or architectural 

 pictures, think that their object-glasses are not subject to any 

 separation of the two foci, and consequently to no variation. 



Phil. Mag. S. 4. Vol. 1 . No. 6. June 1851. 2 L 



