456 Mr. Claudet on a Focus-Equalize?^ for Photography. [May 2, 



A letter from the Foreign Office, addressed to the President, was read, 

 communicating the following paragraph from the ' Moniteur,' transmitted 

 by H. M. Ambassador at Paris : — 



Paris, le 27 Mars. 



" L'Empereur, dans sa sollicitude pour tout ce qui interesse la science 

 et les relations commerciales, a decide que des officiers de la marine et des 

 ingenieurs hydrographes seraient envoy es sur differents points du globe, 

 dans le but de determiner par des observations astronomiques un certain 

 nombre de meridiens fondamentaux qui serviront a assurer la position 

 geographique des lieux inter mediaires. 



" Ce travail important permettra de corriger la Table des latitudes et 

 longitudes inseree dans la Connaissance des temps, et dont les erreurs ont 

 ete signalees dans un rapport addcesse au ministre de I'instruction pub- 

 lique par le president du Bureau des longitudes." 



The following communications were read : — 

 I. " Optics of Photography. — On a Self-acting Pocus-Equalizer,- 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.'^ By A. Claudet, F.R.S. Re- 

 ceived April 8, 1867. 

 "When a solid figure is brought too near the object-glass of a camera 

 obscura, the difference of focus for its various planes is comparatively so 

 great, that it is impossible that all the images should be equally well de- 

 fined. Hence, in the case of photographic portraiture, there is a want of 

 harmony in the representation of the various parts ; some are too sharply 

 delineated, and some others are confused in proportion as they are more 

 and more distant from the plane in focus. But there is another defect which 

 is the consequence of the difference of distance of the various planes bear- 

 ing too great a proportion to the distance of the whole, which is that 

 the nearest parts of the figure are too much enlarged, and the furthest too 

 much reduced. 



In a paper I read at the British Association at Nottingham in 1866, I 

 proposed a plan to obviate these defects, which consisted in bringing all 

 the planes consecutively into focus, by moving, during the exposure, the 

 tube of the lens or the back frame of the camera ; the consequence of which 

 was, of course, that the planes were also during that movement brought 

 out of focus ; so that a sharp image of every plane was impressed upon 

 a confused image; but they were all in the same degree in that mixed 

 state, and the result was an equality of effect producing harmony in the 

 whole, and that kind of softness in the picture so much approved by 

 artists, as resembling, more than the sharpest photographs, the effect that 

 they aim at producing. 



