4 Prof, Petzval on the Camera Obsmra. 



clearly bear no magnifying whatever, but, on the contrary, would 

 require to be inspected at a distance of 12 feet at least ; for it is 

 at this distance that a line ^^th. of an inch in length subtends 

 an angle of 1 minute, and it is only when objects are seen at 

 this visual angle that they appear to be mere points. To obtain 

 a more correct estimate, however, of the sharpness and illumi- 

 nation of the picture in the natural camera, let us compare it 

 with that of a camera with a tolerably good object-glass of 3-inch 

 aperture aud 11-inch focal length. In the middle of the field 

 the pictiu'c furnished by such a camera — there are of course far 

 better instruments — will bear magnifying at least ten times, or, 

 if we may use the expression, will bear inspection at a distance 

 of ^ths of an inch, and consequently, in point of sharpness, is 

 144 -J- 4 = 180 times superior to the picture in the natural 

 camera. With respect to illumination, it will be observed that 

 the two cameras have the same focal length, 11 inches, and con- 

 sequently furnish equal-sized images of all external objects; 

 their apertures, however, have the ratio 1 : 180; that of the 

 first being ^\jth of an inch, whilst that of the second is 3 inches. 

 Now the focal length being constant, the illumination of a pic- 

 ture increases in proportion to the square of the aperture, so 

 that with respect to this property, the camera with, is 32,400 

 times superior to that without glass. It is necessary to observe, 

 however, that a picture so well illuminated as the one here used 

 as a term of comparison, could in practice be scarcely obtained. 

 Two things are worthy of notice in the foregoing. In the first 

 place we see how, by artificial means, that is to say, by means of 

 well-arranged and properly curved lenses, it is possible to in- 

 crease the qualities of sharpness and illumination in an instru- 

 ment, — the first in the ratio of 1 : 180, and the second, indeed, 

 in the ratio of 1 : 32,400. In the second place, we have become 

 acquainted with a kind of aberration which puts a limit to the 

 extreme use of diaphragms before camera lenses. To illustrate 

 this still more, let us suppose that, in order to improve the pro- 

 perties of the picture, we were to try the experiment of reducing, 

 by an interposed diaphragm, the aperture of the lens from 3 

 inches to | an inch. It is evident from the formula (1), where 

 ^~6o;ooo ^'> -A- = 11 in., and /3 = j in., that we should thereby 

 cause the image of a luminous point to become a round spot 

 nearly ygWth of an inch in diameter. Now iu fine engravings, &c. 

 we often meet with lines whose breadth is even less than ^-^oth 

 of an inch ; so that if our blinded lens were employed to copy 

 such engravings, these fine lines would appear still finer in the 

 picture, in consequence of the overlapping of the aberration 

 circles of the adjacent luminous points. This defect would also 

 be increased by the aberrations due to other causes, such as the 



