Prof. Petzval on the Camera Obscura. 9 



to form a plano-convex system. When employed, the convex 

 side is turned towards the object, the plane one towai'ds the 

 image, and the instrument fulfils its purposes more or less per- 

 fectly according as the curvatures of the constituent lenses have 

 been more or less happily chosen. 



In Daguerre's time these telescopic object-glasses, transferred 

 to the camera, were in general use. In all probability, too, they 

 were at first placed in the same manner, with the convex side 

 towards the object ; but experiment must soon have shown that 

 this disposition was not applicable. Tor, destined by their con- 

 struction to give very sharp but very small images, spherical 

 aberx'ation is destroyed only near the axis of such lenses, — in con- 

 sequence of which, when the field of view is larger, a great de- 

 terioration in sharpness is observed on passing from the centre 

 towards the edges of the picture. This deterioration is increased, 

 too, by the fact that the image, instead of being plane, as required 

 by the camera, lies on a curved surface which approaches inform 

 to that of a paraboloid of rotation, whose radius of curvature at 

 the vertex is equal to | of the focal length p. 



In the absence of calculations founded on theory, by means 

 of which the sharpness at the edges of the image might be in- 

 creased, opticians have sought to improve the telescopic lens, so 

 as to adapt it to the camera, by diminishing its superfluous 

 sharpness at the centre, or rather by rendering the contrast 

 between the centre and the edges less striking. To obtain a 

 notion of how this may be accomplished, let the object-lens of a 

 good telescope be unscrewed and turned so as to present its plane 

 side to the object. By so doing, the good telescope will be con- 

 verted into a very poor instrument ; and in order to obtain even 

 a tolerable image, extreme blinding of the lens must be resorted 

 to. The reason of this is to be sought in the serious spherical 

 aberration that has been called into existence ; the rays belonging 

 to one and the same cylinder no longer converge towards a single 

 point, but by their successive intersections give rise to a lumi- 

 nous curve or caustic, whose path intersects all the planes drawn 

 perpendicular to the axis in the vicinity of the focus, by which 

 latter term is now meant the point to which rays parallel to, and 

 venj near the axis, converge. The advantage of a diaphragm 

 before the lens is, that it can be placed so as to admit only those 

 rays of a cylinder whose intersections correspond to that part of 

 the caustic which is situated in the plane of the picture. In 

 order to convert a telescopic lens into a tolerably good camera 

 lens, the cylinder of rays which corresponds to an image near 

 the edge of the field must be treated in the manner described, 

 and the position of the diaphragm determined accordingly. With 

 a lens 3 inches in aperture and a focal length of 16 inches, such 



