for measuring the Intensity of the Photogenic Rays. 487 



neutralize the photogenic action in the centre of the lens in a 

 different proportion. 



Therefore two different lenses of the same cui'vatures, and of 

 the same density and dispersion, may not manifest the same 

 degree of chromatic correction ; and moreover, the chromatic 

 correction may comport differently in each glass according to the 

 variation in the colour of the light combined with the colour or 

 other properties of the glass itself*. 



From the preceding observations it follows, that the colour of 

 the glasses, the degree of their chromatic aberration m the 

 several points of the lens, may produce the same result ; for 

 according to the chromatic correction of these vaiious points, 

 more or less yellow rays and more or less photogenic rays may 

 be concentrated from the centre to the circumference, and the 

 neutralizing action of the yellow rays may consequently be 

 greater in the centre of one lens than in the centre of another. 

 Then the separation of the two foci, in two object-glasses, may, 

 by the influence of the same light, undergo a change in an ano- 

 malous direction. 



The general colour of light in different climates may also 

 modify the achromatism of the photogenic rays ; and for this 

 reason the two foci may be more or less separated in Vienna or 

 Paris than in London, and a great difference may be observed 

 in using the same lenses in Eiu'ope or in America. We should 

 therefore be exposed to great errors if we were to be guided by 

 the fixed mode of compensation indicated beforehand on the tube 

 of the object-glass; and it is impossible to dispense with ascex'- 

 taining frequently the exact position of the photogenic focus by 

 means of the focimeter. 



It is important to consider that the mathematical focus exists 

 only for one plane ; nevertheless the objects placed beyond and 

 within that plane in a moderate limit can be represented with 

 sufficient correctness. It is therefore possible to obtain a pass- 

 able image at some distance from the mathematical focus, but that 

 distance must not be too great. In taking portraits, the objectr 

 glass must represent at the same time parts situated in various 

 planes, in an interval of at least two feet. It is impossible to 

 have an exact focus for all these parts ; but none must be too 



* Every one must have observed that vei-y often it is impossible with the 

 best objet't-glasses to obtain on the ground glass a well-defined image ; and 

 tliat ut other times, under diif'ereut conditions of light, a perfect image is 

 easily obtained. It is so with the pliotogenic image ; sometimes every seg- 

 ment of the focimeter a])pears well-defined, and at others they appear 

 all confused ; consecpiently there are some days when it is im])ossible to 

 obtain satisfactory results, and others when we succeed every time. It ii 

 therefore evident that the achruiuatism uf ubjeet-glu8ses experiences iuce^r 

 sant variations. 



