TRANSACTIONS OF THE SECTIONS. 63 



beautiful principle of Woodward's apparatus consists in his having decided the ques- 

 tion of the position of the focus of the condenser, 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 has undertaken, in the interest of the photographic 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 illu. 

 minating the negative. The solar camera does not require any diaphragm to reduce 

 the aperture of the lens, because every one of the points of the negative are visible 

 only when they are defined on the image of the sun, and they are so exclusively for 

 the centre of the lens, the only point which sees the sun ; while the various points 

 of the negative, which from 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 produced 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 nega- 

 tive. It is evident that from the centre of the lens the whole negative has for back- 

 ground the sun itself, and from the other points of the lens it has for background 

 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 construction. 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 minror should be capable, by its connexion with a heliostat, of following 

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

 refract the image of the sun without dispersion, and to define more correctly the 

 lines of the negative ; and a no less important condition for losing nothing of the 

 photogenic rays would be, to have the condenser formed with a glass perfectly 

 homogeneous and colourless. With such improvements, the solar camera will be- 

 come capable of producing results of the greatest beauty ; and, without any question, 

 its introduction into the photographer's studio will mark a period of considerable 

 improvement in the art. 



On a Reflecting Telescope for Celestial Photography, erecting at Hastings, 

 near New York. By Henry Draper, M.D. 



In the summer of 1857, after the Dublin meeting of the British Association, a 

 party visited Lord Rosse's telescope at Parsonstown. We were shown the ma- 

 chinery employed in its construction, and, as far as the weather permitted, its per- 

 formance. 



That visit first led me to attempt constructing an instrument which should be 

 specially adapted for celestial photography, for which purpose the reflector possesses 

 such conspicuous advantages over any refractor. 



Those who are familiar with photographic operations know well how important it 

 is for the ensuring of uniform success, that the sensitive surfaces should always be 

 placed in similar circumstances as to position, and that position must afford every 

 facility for carrying on the necessary manipulations. 



It appeared to me that a modification of a form of mounting, proposed some 

 time ago by Mr. Nasmyth, could be made to answer these requirements perfectly, 



