Prof. Petzval on the Camera Obscura. 13 



is true approximately for every other cylinder inclined to the 

 axis of the instrument. Ai'ound the centre of the second lens, 

 therefore, let us conceive a circle of A inch radius described ; 

 its circumference will be at a distance of 1 inch from that of the 

 lens, and it is clear that the second lens will admit all the rays 

 of every cylinder whose inclination to the axis of the instrument 

 is such, that the axial ray of that cylinder, after passing unre- 

 fracted through the first lens, meets the second in the circum- 

 ference of the above circle. The image produced by such a cy- 

 linder, therefore, will possess the same maximum of illumination 

 as do the central images. But the entrance of the rays of other 

 cylinders more inclined to the axis of the instrument will be 

 more or less impeded ; and by following the method already ex- 

 plained in the case of Daguerre's camera, it will be found that 

 throughout a field of 10|° there will be maximum light ; that 

 between this and a field of 32" the intensity of light will dimi- 

 nish to half its normal value ; and lastly, that the whole extent 

 of the field, beyond which is darkness, amounts to about 50°. 

 These angles correspond on the picture to circles whose diame- 

 ters are 2, 6, and 10 inches respectively. 



When portraits only are to be taken, that is to say, when a 

 correct picture of only a small portion of the object is desired, 

 this unequal distribution of light is of no great importance. In 

 the case of landscapes, however, it forms a serious defect, and 

 necessitates the use of diaphragms, not only to distribute the 

 light more uniformly, but also to diminish the influence of the 

 unequal distances of objects, and to soften the curvature of the 

 image. The best place for the diaphragm is exactly midway 

 between the two lenses, and by diminishing the intensity of light 

 to ith, ith or to Y^^thof its full value, the field of equal illumination 

 may be increased to 31°, whilst the two zones, wherein the light 

 first diminishes to half its normal value and then to zero, may be 

 made much narrower. On finding that the picture thus obtained 

 was superior to that of a camera with a simple achromatic lens, 

 the instrument, which was constructed for portraits, was em- 

 ployed also for landscapes; and larger pictures being desired, 

 the original object-glass of l,j inch aperture was reproduced 

 on a larger scale, the aperture being increased to 3, 4 and even 

 5 inches in order to obtain pictures of 14 inches diameter. 

 Practical opticians undertook this increase in size on their own 

 responsibility ; and the necessity of applying certain con-ections 

 to the curvatures was not attended to ; the consequence of which 

 was, tliat the later productions of the camera were in every re- 

 spect incomplete, and deteriorated by spherical aberration, double 

 foci, and other imperfections. These efforts to increase the size 

 of the original instrument being in other respects unpromising, 



