486 M. A. Claudet on the Dynactinometer, an Imtrurnent 



The extraordinary lengtli of the photogenic spectrum, and its 

 various degrees of refrangibility, which render the achromatism 

 of the whole very difficult, are proved by anomalous effects 

 taking place in the formation of the photogenic image. All 

 photographers must have observed, that when the exposure in 

 the camera-obscura has been too long, if the whole aperture of 

 the lens has been employed, the image prodiiced is rarely very 

 well defined. The reason of it is, that the extreme photogenic 

 rays, which are the weakest, have been brought into action, and 

 these have produced a confusion by their dispersion ; but had the 

 exposure been shorter, these extreme rays would have remained 

 inactive, and the definition would have been more perfect. 



But when the yellow rays predominate, owing to media 

 absorbing a proportion of the most refrangible rays, then the 

 central part of the lens, from which the yellow rays are more con- 

 densed with the photogenic rays, will have less power on account 

 of the neutralizing action of the excess of yellow rays. In this 

 case the centre will operate less or not at all, and the foci will 

 be the less distant, because the visual image will be formed, as 

 before, by the less I'efracted rays from the whole aperture, and 

 the photogenic image by a smaller proportion of the most refran- 

 gible, which would have become less refracted than the visual 

 rays by the same over-correction, and consequently the conver- 

 gence of the whole will be more parallel. All absorbing causes 

 contract the length of the spectrum or reduce the dispersion of 

 the rays. 



In lenses having their photogenic focus longer than the visual 

 focus, it is obvious that this effect is produced by an over-correc- 

 tion of the photogenic rays, and the greater or less separation of 

 the two foci must follow the law I have just described. But for 

 lenses in which the over-correction has not taken place, and with 

 their photogenic focus shorter than the visual focus, the same 

 cii'cumstances which separate more and more the two foci in the 

 former case must render them more coincident in the second. 

 Other circumstances are capable of modifying that phsenomenon 

 in several ways. 



In some lenses, owing to their degree of chromatic correction 

 different on the several points of their curvatures, the same 

 conditions of light will produce different effects ; and this ex- 

 plains the anomaly I had observed in my fii'st memoir, that 

 the separation of the two foci follows different and even contrary 

 laws in different lenses, and in lenses apparently similar. The 

 colour of the glass itself may occasion in one lens a greater 

 absorption of certain rays than in another. If the glass of one 

 lens absorb more or less photogenic rays than the other, and 

 by its colom- produce more or less yellow rays, these last will 



