Prof. Petzval on the Camera Obscura. Il 



whose axes exactly graze the edge of the lens will be admitted by 

 the latter, the entrance of the rest being prevented by the setting. 

 These rays are inclined at an angle of 26° to the axis of the in- 

 strument, so that between 36° and 53^ the intensity of light 

 in the field will diminish from its maximum value to one-half of 

 the same. Lastly, the lens will admit none of the rays of the 

 cylinders whose axes meet its plane at a distance of half an inch 

 from its edge ; consequently, between 52° and 66° the intensity 

 of light diminishes from half its normal value down to zero. 

 Thus when uniform light is required, the field must not exceed 

 36° ; in other words, the focal length being 16 inches, the dia- 

 meter of the circular picture cannot exceed 10 inches. 



Such are the properties of the instrument with which Dagueri'e 

 worked when he made his beautiful discovery. At that time 

 silver plates, coated with iodine, were alone employed ; and the 

 time of exposure required was so great — half an hour — that 

 portrait-taking was next to impossible. Hence arose the demand 

 for a camera lens producing greater illumination, and equal or, 

 if possible, greater sharpness. Sooner or later practical opti- 

 cians would, no doubt, have sought to improve the camera of 

 Daguerre by substituting a convex-concave, in place of the plano- 

 convex achromatic lens ; for the former, treated in the manner 

 above described, possesses several advantages. Science, how- 

 ever, stepped in with more efiicient means, and Prof. Petzval, 

 after a thorough theoretical investigation of the subject, set 

 about constructing his first object-glass, destined principally for 

 portrait-taking. 



In so doing he was guided by the following considerations : — 

 The object-lens of a telescope has only three conditions to fulfil : 

 first, to possess a given focal length ; second, to be achromatic ; 

 and thh-d, to reduce the spherical aberration to a minimum. The 

 first is a matter of small importance, for within certain limits 

 the focal length may vary; the achromatism depends on the 

 focal lengths of the constituent lenses, and the spherical aberra- 

 tion on the curvatures of their surfaces. The three conditions, 

 therefore, can be fulfilled by suitably disposing of three optical 

 elements, i. e. the curvatures of three surfaces. 



In the camera, however, the number of these conditions is 

 raised from three to eight, five of which have reference to a much 

 more complete destruction of spherical aberration, two to the 

 production of achromatism, and the eighth to the position of the 

 focus. Instead of three, therefore, eight optical elements are 

 requisite, the choice of which will be determined by the follow- 

 ing considerations. Greater illumination, one of the desired 

 improvements, can only be obtained in two ways — by enlarging 

 the aperture and by diminishing the focal length, both which, 



