CHEMICAL SCIENCE. 197 



its discovery. By its means small negatives may produce pictures 

 magnified to any extent ; a portrait taken on a collodion plate not 

 larger than a visiting card can be increased, in the greatest perfection, 

 to the size of nature ; views as small as those for the stereoscope can 

 be also considerably enlarged. 



The beautiful principle of Woodward's apparatus consists in his 

 having decided the question of the position of the focus of the con- 

 denser, and in having placed it exactly on the front lens of the 

 camera obscura. » 



As this principle had not yet been explained when the invention 

 was exhibited before the Photographic Societies of London and 

 Paris, and not even by the inventor himself in the specification of 

 his patent, Mr. Claudet undertook, in the interest of the photo- 

 graphic art, to bring the subject before the British Association, and 

 to demonstrate that the solar camera of Woodward has solved the 

 most difficult problem of the optics of photography, and is capable of 

 producing wonderful results. This problem consists in forming the 

 image of the negative to be copied only by the centre of the object- 

 glass reduced to the smallest aperture possible, without losing the 

 least proportion of the light illuminating the negative. 



The solar camera does not require any diaphragm to reduce the 

 aperture of the lens, because every one of the points pf the negative 

 are visible only n hen they are defined on the image of the sun, and 

 they are so (in that position exclusively), for the centre of the lens is 

 the only point which sees the sun, while the various points of the 

 negative which form the marginal zone of the lens are defined 

 against the comparatively obscure parts of the sky surrounding the 

 sun, are, as it were, invisible to that zone ; so that the image is pro- 

 duced only by the central rays, and not in the least degree by any 

 other points of the lens, which are subject to spherical aberration. 

 It is, in fact, a lens reduced to an aperture as small as is the image 

 of the sun upon its surface, without the necessity of any diaphragm, 

 and admitting the whole light of the sun after it has been condensed 

 upon the various separate points of the negative. It is evident that 

 from the centre of the lens the whole negative has for background 

 the sun itself, and from the other points of the lens it has for back- 

 ground only the sky surrounding the sun, which fortunately has no 

 effect in the formation of the image. 



Such is the essential principle of Woodward's solar camera, which 

 did not exist in that instrument when the focus of the condenser was 

 not on the object-glass. This principle is truly marvellous, but it 

 must be observed that the solar camera, precisely on account of the 

 excellence of this principle, requires the greatest precision in its con- 

 struction. For its delicate performances, it must be as perfect as an 

 astronomical instrument, which, in fact, it is. The reflecting 

 mirror should be plane, and with parallel surfaces, in order to reflect 

 on the condenser an image of the sun without deformation ; and in 

 order to keep the image always on the very centre of the object-glass, 

 the only condition for the exclusion of the oblique rays, the mirror 

 should be capable by its connexion with a heliostat of following the 

 movements of the sun. The condenser itself should be achromatic, 



