14 Prof. Petzval on the Camera Obscvra. 



the demand arose for a new object-glass, which, without sup- 

 planting the old one, but rather restricting the same to the use 

 for which it was intended, should be suited to the reproduction 

 of landscapes, maps, engravings, &c. 



The modifications applied to the old instrument to fit it for 

 its new purposes, had for their object, principally, to increase the 

 magnitude of the field and the uniformity of its illumination, and 

 consisted in a diminution of the distance between tlie two lenses, 

 and of the aperture of the second. The object-glass, constructed 

 carefully with a view of fulfilling all the new conditions, and 

 submitted to the Academy of Vienna, consists, as before, of two 

 lenses ; the first has an aperture of 3 inches and the second of 

 of 2 inches, the clear distance between the two being 1 inch. 

 The magnitude of the picture is the same as that corresponding 

 to a single achromatic lens of 26 inches focal length, its diameter 

 being 20 inches ; in other words, the field amounts to 42° and 

 is uniformly lighted. This last result is due to the diminished 

 aperture of the second lens, and has been purchased, of course, 

 at the expense of intensity of light. The curvature of the image 

 of a plane object is small, its radius at the vertex being about 

 80 inches. 



With respect to the achromatism of the two lenses, it is well 

 known that the ratio between the indices of refraction for crown- 

 and flint-glass is not constant, but varies with the colour of the 

 ray, and that on this account the rays of all colours cannot be 

 made to coincide, simultaneously, by any arrangement of the two 

 kinds of glass ; in other words, according to the technical ex- 

 pression, a certain chromatic aberration of the secondary spectrum 

 always remains. In the telescope most attention is paid to the 

 coincidence of the rays at the red end of the spectrum, and, 

 without injury to the picture, a considerable aberration of the 

 rays at the violet end may exist. These rays, however, exert the 

 greatest chemical action, whence it happens that the object-lena 

 of a telescope gives a less sharp photographic, than it does an 

 optical image. On the other hand, if the opposite end of the 

 spectrum were most attended to, the photographic picture would 

 be improved at the expense of the optical one, and in both cases 

 the chemical and optical foci would be separated. In con- 

 structing the new object-glass, the whole spectrum, rather than 

 either end of the same, was regarded, and the most active che- 

 mical made to coincide, approximately, with the most active 

 optical rays, so that, for a healthy eye, the chemical and optical 

 foci coincide. 



From the above exposition it follows that, whilst the new 

 camera is inferior to the old in point of illumination, it far sur- 

 passes the latter in magnitude of the field, and in uniformity of 



