Mr. A. Claudet on the Optics of Photography. 215 



rable that only one part of the face should be in that condition, 

 while all the others should gradually lose their sharpness as they 

 are more and more distant from the plane of definition. 



We can at will bring the focus upon any plane of the figure. 

 In taking portraits not smaller than miniature size, we may 

 choose the nose, the eyes, or the ears; but we cannot have 

 the three equally sharply defined ; and photographers endeavour 

 to focus upon a middle plane, for example upon the eyes, in 

 order to have the nose and the ears in the same degree of less 

 perfect definition, not very far from that of the eyes. 



Perfection in the portrait would be attained, were it possible 

 to do so, first by taking the image of the nose, then, after having 

 altered the focus, the image of the eyes, and finally, after again 

 altering the focus, the image of the ear, and then, from these 

 various images, forming a collective portrait. Such an idea may 

 appear impracticable, possibly even absurd, and it is sure on first 

 thoughts to be rejected and condemned. Yet I seriously, and 

 after mature consideration both of the practice and of the theory 

 of such a scheme, propose its adoption as one of the greatest im- 

 provements which will have been introduced in photography since 

 its discovery. I beg to be allowed to explain the method in which 

 I conceive I have solved the difficulty I have above alluded to. 



Let me premise a few words upon the effect produced by the ex- 

 periment of taking the photographic image of the focimeter. This 

 instrument, I may be permitted to remark, was invented by me 

 upwards of twenty years ago, and has been constantly used in my 

 operating-room in order to test in wmat degree the chemical and 

 visual foci of lenses coincided or differed. Until it came into 

 use, nobody had ever dreamt that they did not exist in the same 

 plane when the object-glasses were as much achromatic as those of 

 the best telescopes. This fact being demonstrated by the instru- 

 ment I refer to, was the cause of a complete change in the con- 

 struction of lenses for photographic purposes ; and from that time 

 opticians have endeavoured to calculate, and succeeded in disco- 

 vering curvatures the combination of which, to invent a phrase, 

 achromatize, with the visible rays of light, the invisible rays which 

 are exclusively endowed with the chemical action. The use of the 

 focimeter I have found indispensable since the further discovery 

 I have made that the two foci undergo continual changes from 

 various atmospheric influences; and no photographic studio, 

 therefore, should be without this instrument; for no optical 

 combination is capable of preserving an invariable coincidence of 

 foci, and the photographer must have the means at any moment 

 of testing the then state of the elements, and of the light itself, 

 in order to ascertain any change in its refraction and to act ac- 

 cordingly. 



