12 Prof. Petzval on the Came^-a Obscura, 



however, will result from employing two converging lenses 

 instead of one. These lenses must of course be achromatic ; and 

 by theory, in order that a good image may be produced, they 

 must be separated from each other by a distance not less than 

 one-third of the focal length of the lens next the object. In 

 order to form the eight requisite elements, therefore, seven lens 

 surfaces and one distance may be selected. By this selection 

 the first lens need but present three surfaces to be disposed of, 

 so that its constituents may have a common surface ; the second 

 lens, however, in order to furnish the remaining four surfaces, 

 must have its constituents separated, even though by so doing 

 light is lost. 



In accordance with these data. Prof. Petzval calculated, and 

 Voigtlander constructed, a new object-lens which had an aper- 

 ture of li inch, and a focal length of 5| inches. With it por- 

 traits were taken in forty seconds ; in point of illumination it 

 was sixteen times superior to the camera of Daguerre, and its 

 images were sharp enough to bear magnifying twenty times. 

 The principal defects of the new camera were a curved image 

 and limited field of view, both of which resulted from the em- 

 ployment of separated lenses. 



With respect to the first defect, the image of a plane object 

 was, according to theory, situated in the hollow of a paraboloid 

 of rotation, having at its vertex a radius of curvature equal to 7 

 or 8 inches. In object-glasses afterwards constructed, where 

 the aperture was increased to 3 inches, this curvatiu'e M'as soft- 

 ened to 15 inches. By sacrificing a little sharpness at the edges, 

 too, circumstances generally furnished means of softening this 

 curvature still more. For portraits, indeed, a camera capable of 

 giving a plane image of a plane object would be no acquisition, 

 inasmuch as the persons whose portraits are to be taken by no 

 means constitute such plane objects. With a single individual, 

 or Avith a group of such, the skilful photographer may always 

 arrange the position of his subjects so that the image will fall 

 nearly in a plane. 



The second action of the separated lenses deserves closer ex- 

 amination. It will be at once seen that here the setting of the 

 first lens plays the part of the former diaphragm, and modifies 

 the admission of light to the second lens. As an example, let 

 us take an object-glass whose two lenses are 5|^ inches apart, the 

 aperture of each being 3 inches. Let the focal length of the first 

 lens be 16 inches, and that of the second 24 inches. Then by 

 means of the first lens, a cylinder of rays parallel to the axis be- 

 comes converted into a cone, whose vertex is 16 inches behind 

 this lens ; and the plane of the second lens intercepts this cone 

 in a circle Avhose diameter is diminished to 2 inches ; the same 



