Ixxxvi 



quisite detail of the Daguerreotype. Ke assisted Sir Charles Wheatstone 

 in the early application of the stereoscope to photography ; and in his 

 admirable treatise on the stereoscope he gives the history of the art and 

 the theory of the principles of binocular vision. His great aim was the 

 elevation of photography by rendering her work scientifically true ; and 

 the Reports of the British Association during a period of twenty years 

 bear ample testimony to the ingenuity and originality of his inventions. 

 His dynactinometer, his photographometer, his focimeter, his stereomono- 

 scope, his system of unity of measure for focusing enlargements, his system 

 of photosculpture, and other results of his experimental researches are fa- 

 miliar to most photographers. 



In the later years of his life he became convinced that one of the greatest 

 deficiencies of photography, in the representation of solid figures, is the in- 

 capability of obtaining an equally well-defined image of all the various parts 

 situated on different planes. Hence it became his object to remove from 

 photographic portraiture the mechanical harshness which marked and 

 marred the plane situated in the exact focus of the lens, and so to pro- 

 duce, as in the best works of art, a uniformly soft and harmonious treat- 

 ment. His success in the first instance was partial, inasmuch as the 

 adopted motion of the posterior lens only of the optical combination slightly 

 altered the size of the superimposed images, and thus introduced a theo- 

 retical, though hardly visible, amount of blurring. Dr. Sommer, M. Voigt- 

 lander's stepson, supplied a series of formulae showing that, although for 

 all practical purposes in photography the movement of one lens attained 

 the object in view, yet the simultaneous motion of the two lenses, receding 

 from or approaching a fixed point between them, was the only legitimate 

 mode of reconciling practice with theory, and of securing in every plane an 

 exact uniformity of image. To fulfil this condition was a difficult pro- 

 blem, the solution of which was most perplexing. But, says Claudet, with 

 a determination which commands success, " I did not like that it should 

 be said my plan was not entirely in accordance with the mathematical laws 

 of optics, and I set to work to find a mechanical means by which I could 

 avail myself of the calculations of Dr. Sommer. I have found such means'* 

 and it proves that the differential movement can be effected, not only as 

 readily, but with a greater command and steadiness than by moving only 

 one lens." His ingenious automatic arrangement is described in his last 

 paper read before the Royal Society, in 1867, and published in the Pro- 

 ceedings, entitled "Optics of Photography : on a Self-acting Focus-Equa- 

 lizer, or the means of producing the Differential Movement of the two 

 Lenses of a Photographic Optical Combination, which is capable, during 

 the exposure, of bringing consecutively all the Planes of a Solid Figure into 

 Focus, without altering the size of the various images superposed." 



After this, and in the same year, he had an interesting correspondence 

 with his veteran collaborates Sir David Brewster, who held that the most 

 perfect 'photographic instrument is a single lens of least dispersion, and 



