for measuring the Intensity of tlie Photogenic Rays. 485 



In comparing the difference between the two foci given by 

 rays refracted by several points of any object-glass, I have found 

 that the difference is greater for the parts of the object-glass the 

 most distant from the centre. This must be due to the imper- 

 fection of the chromatic correction, which imperfection is greater 

 for the most oblique pencils than for those which become gra- 

 dually more parallel, as they are refracted from parts nearer 

 the centre of the lens. 



There is a certain pi-oportion of all the rays of the spectioim, 

 which, when combined, possess as great a photogenic power as 

 the pm-e insulated photogenic rays. This condition happens 

 when the atmosphere is pure and the light perfectly white. 



In these circumstances, the proportion of yellow rays is in its 

 minimum compared with the emission of photogenic rays ; and 

 the concentration of all the rays on the same point gives as great a 

 photogenic power as the pure photogenic rays alone. In this case 

 the centre of the lens will operate as much as any distant point, 

 where the photogenic rays are more dispersed from the yellow,. 



Then the two foci will appear the most separated, because the 

 whole aperture -svill contribute to the formation of the photogenic 

 as well as the visual image ; the photogenic action being produced 

 by the greatest proportion of the more refrangible rays, which, 

 by the over-correction, have become less refracted than the visual 

 rays. 



These phsenomena will be better understood if we consider, 

 that the photogenic space of the spectrum extends from the 

 green ray and considerably beyond the violet, and that the same 

 photogenic space is longer or shorter in proportion to the purity 

 of the atmosphere. Any absorbing causes, at all events those 

 which obscure the atmosphere, begin first to neutralize the most 

 refrangible extremity ; and as these causes increase in intensity, 

 the photogenic space is gradually reduced in following the dimi- 

 nution of refrangibility, in such a manner that the violet rays 

 are absorbed befoi'c the indigo rays, and these last before the 

 blue. But none arc absorbed all at once ; they are gi'adually 

 extinguished as the absorbing medium becomes thicker. The 

 photogenic rays ai-e subject to the same law ; the most refran- 

 gible part disappears the first. During this process of absoi-p- 

 tion, the mean refrangiljility of the photogenic space so reduced 

 approaches nearer and nearer the mean refrangibility of the visual 

 rays ; so that the achromatism of the whole specti-um becomes 

 more and more perfect as its lengtli diminishes. This effect is 

 not veiy conspicuous when the visual rays only are in question ; 

 but when three-fourths of the long photogenic space have been 

 absorbed, it is evident that the conditions of achromatism must 

 have experienced a considerable modification. 



