for measuring the Intensity of the Photogenic Rays. 479 



the shadows by another, consequently no colours are repre- 

 sented. 



The luminous rays being endowed with various degrees of 

 refrangibility, are, on an average, considerably less refracted than 

 the actinic rays. Hence the difficulty, by means of the process 

 of achromatism, to bring them all to the same focus. An object- 

 glass, although perfectly achromatic for the luminous rays, may 

 give a visual image at a focus which does not coincide with the 

 focus of the actinic rays. 



For a long time it was supposed that achromatic lenses repre- 

 sented the visual image at the same focus as the actinic image j 

 and this supposition was a continual source of eri'or in the 

 result of photographic operations. 



In the year ISM, I was induced to investigate this question 

 on account of the difficulty I had continually experienced, since 

 the beginning of photography, to obtain well-defined photo- 

 graphic pictm-es with the best achromatic lenses giving the most 

 perfect images on the ground glass. 



After a long series of experiments, I discovered that in achro- 

 matic lenses generally the photographic focus does not coincide 

 with the visual focus. I communicated a paper on this subject 

 to the Royal Society, and to the Academic des Sciences. 



I had at first much difficulty to convince opticians and photo- 

 graphers that there was such a difference in achromatic lenses. 

 But M. Lerebours of Paris, at my suggestion, having investigated 

 that question, acknowledged the correctness of the fact, explained 

 the cause of the difference, and indicated the means of avoiding 

 it, or correcting the lenses subject to it. 



This point seems now well established ; and opticians, by new 

 formulae, have changed the curvatures of their lenses, in order to 

 construct object-glasses having their two foci coincident. They 

 nearly all announce that the lenses they now produce are exempt 

 from any difference, and that they can correct those they had con- 

 stnicted before my discovery. A few others, rightly considering 

 that there is no practical advantage in lenses having their foci 

 coincident, in order to coiTcct the difference, mark the sli- 

 ding tube of the object-glasses with small divisions, to which, 

 after having set the focus on the ground glass, it is necessary to 

 pull in or out the tube according to the distance of the object. 

 Although this mode of correction is not exact in all circum- 

 stances, as I shall hereafter explain, still by it they fully acknow- 

 ledge the tnith of the fact which had escaped them before. 



But I have lately discovered another fact, as important and 

 as extraordinary, viz. that, by some causes which cannot yet be 

 explained in a satisfactory manner, there is a constant variation 

 in the difference between these two foci. 1 have indicated the 



