218 Mr. A. Claudet on the Optics of Photography, 



focus of every plane to the immoveable frame holding the plate ; 

 and we are enabled thus to represent consecutively on the plate 

 an image of every plane, with a less reduction or increase of size 

 than when the power of the double combination remains the 

 same; for it happens fortunately that, to reduce the focus, we 

 must separate the lenses, by which the power is increased. The 

 alteration of the distance which separates the two lenses is effected 

 by a rack and pinion acting upon a tube containing the back 

 lens, that tube sliding into another containing the front lens, 

 which remains fixed during the adaptation of the focus to the 

 distance of every plane by means of a gradual movement com- 

 municated to the back lens during the sitting. The inspection 

 of the apparatus, which T submit to the Meeting, will enable 

 any visitor interested in the question to understand its action. 



It is marvellous when we reflect that there is nothing to wish 

 for in the shape of contrivances having for their object the percep- 

 tion of vision, and that from time to time man invents, or thinks 

 he invents, what nature had done in the most perfect manner. The 

 eye is supplied with a lens in the same way as the camera obscura ; 

 the retina is the screen on which, like the ground glass of the 

 camera, the light reflected by all the natural objects form their 

 image. By various humours through which the light is refracted, 

 the spherical aberration is corrected and the most perfect achro- 

 matism is produced; the eye is endowed with muscles which 

 enable it to alter the focal distance of the lens according to the 

 various distances of the objects. Optics is able to imitate all 

 these beautiful contrivances except the last, which is available 

 only on account of the way in which we exercise the perception 

 of vision. We see at once only a very small part of the image — 

 that part which is projected on the centre of the retina; and the 

 eye can adapt its focus to the distance of that part, and, as ra- 

 pidly as thought, when directing its attention to another part it 

 adapts its focus to that new distance. Therefore it matters not 

 whether the other parts are in focus; we have only the perception 

 of what we want to see, and, by the proper adaptation, that sensa- 

 tion conveys to our mind only a well-defined image. It cannot 

 be so with the camera obscura, because, the photographic image 

 produced by it being at once permanently fixed entire by the 

 same exposure, we cannot change it in changing the focus ; the 

 only thing we can do is to impress a stronger image on a fainter 

 image. The artificial optical instrument being destitute of a 

 self-acting changing adaptation to the focus of all the other 

 planes, can represent only one plane in focus; but if it had that 

 adaptation, the surface receiving the impression of the image in 

 a permanent manner (not like the retina, which does not retain 

 the impression), that impression would consist of a number of 



