2 Prof. Petzval on the Camera Obscura. 



behind a small hole in the shutter of a carefully darkened room, 

 an inverted image of external objects is at once obtained which 

 possesses, in great perfection, many of the desired properties. We 

 have here absolute faithfulness to nature ; pictures at once of 

 near and of distant objects, a field of vision as near to 180° as 

 wc please, and either a plane or a curved image. The expense 

 of such an apparatus is small enough, and its convenience indis- 

 putable : in short, the picture obtained fails only in sharpness 

 and illumination, but it must be admitted that these defects are 

 so serious as to render the arrangement next to worthless for most 

 purposes. Nevertheless, for many reasons, the arrangement in 

 question deserves closer examination : it fm-nishes an excellent 

 example of what nature presents, and of what art must supply ; 

 we learn from it also how often natural endowments arc sacrificed 

 when, by artificial means, we seek to enhance the nobler proper- 

 ties of sharpness and illumination ; and lastly, we may here study 

 the nature and influence of the imperfections inseparable from 

 this the natural camera. 



Let us assume that the external object is so distant, that eveiy 

 point of the same sends to the hole in the shutter a cone of rays 

 so acute as not to differ essentially from a cyHnder. If light 

 were propagated in straight lines, it is manifest that the rays of 

 every such cylinder would reach the screen in full possession of 

 their own peculiar colour and intensity of light, and that they 

 would impart both these qualities to a small portion of that 

 screen, nearly circular in form, and of the same size as the hole. 

 The several coloured spots thus formed would group themselves 

 so as to constitute an inverted picture of the object, and the 

 sharpness of this picture would be capable of being augmented 

 indefinitely by diminishing the size of the hole. 



Light, however, instead of being propagated in straight lines, 

 is turned aside or diffracted on passing through an aperture, and 

 thus gives rise to far different phsenomena. The external object 

 being a luminous point, a star for instance, its image is not 

 only always greater than the hole, but on diminishing the size 

 of the latter we find that, as soon as a certain limit has been 

 reached, the image, instead of diminishing accordingly, actually 

 becomes larger and less luminous. On closer examination this 

 image is found to consist of a round luminous spot surrounded 

 by concentric rings, alternately light and dark. The central 

 spot is always found to possess the greatest intensity of light, 

 the surrounding light rings being in general so faint as only to 

 be perceptible by artificial means. 



Since in everything which concerns the telescope, the micro- 

 scope, or the camera, it is of the utmost importance to study the 

 nature and magnitude of the defects caused in the picture by the 



