Prof. Petzval on the Camera Obscura. 15 



sharpness as well as of illumination. Whilst the new camera, 

 therefore, is best adapted for landscapes, the old one may still be 

 used whenever a brief period of exposure is desirable, as in 

 taking pictures of living animals *. 



To suit the properties of the new object-glass, a new camera, 

 greatly exceeding the ordinary ones in bulk, became necessary. 

 M. Petzval submitted a design to the Academy, remarking at the 

 same time that it was no doubt susceptible of improvement. 

 Without entering into a detailed description of all the arrange- 

 ments in this camera, it will suffice to note the principal objects 

 aimed at. These were a diminution of the whole mass as much 

 as possible, a division of the same into several convenient parts, 

 avoidance of false light, an arrangement for inclhiing the plane 

 of the image to the axis of tlie instrument, in order to accom- 

 modate the diiferent distances of the objects, and lastly, an im- 

 proved method of uncovering the iodized plate without shaking 

 the instrument. 



Whilst admitting that his attempt to attain these objects is 

 susceptible of being improved upon. Prof Petzval does not 

 hesitate to pronounce his object-glass to be the best attainable 

 with the given optical elements. In order to test its merits, he 

 selects the most delicate of all photographic problems, that of 

 obtaining, on a reduced scale, the copy of a map or engraving. 

 He enters into interesting details as to the best method to 

 be employed, the principal feature of which consists in first 

 bending the plane of the map to be copied into a developable 

 surface, so as to make it approach as much as possible to coin- 

 cidence with the parabolic surface, which according to theory, 

 corresponds to a plane image. As an example, he takes a map 24 

 inches by 16 inches, and places his instrument at a distance of 

 13 feet, in order to obtain a copy ith the size of the original. 

 Every expedient being adopted, he calculates then that all lines 

 in the original whose breadth exceeds aroth of an inch will be 

 transferred to the copy. 



After some remarks on obtaining enlarged copies of pictures, 

 and a few instructions for testing the merits of an object-glass, 

 Prof. Petzval concludes his valuable report by stating that he 

 has confided the construction of his new object-glasses to M. C, 

 Dietzler. 



* The new camera has also the advantage of having a more invariable 

 period of exposure, that of the old one depending greatly on the magni- 

 tude of the field. Approximately, however, these periods have the ratio of 

 3: 1. 



