8 Prof. Petzval on the Camera Obscura. 



ference between a and p, or the nearer the object. To take an 

 extreme case, by way of example, let a = 2p, that is to say, let 

 the distance of the object be reduced to double the focal length, 

 then by (5) a = 2p, and by (6) da.=Adp; so that the linear 

 chromatic aberration is now quadrupled, and, as a consequence, 

 the distance between the separated foci is increased to an inch. 

 This varying distance between the chemical foci constituting, as 

 it does, so serious a defect inseparable from all cameras with un- 

 achromatic lenses, the best possible achromatism is even more 

 indispensable for this instrument than it is for the telescope 

 itself. 



The formula (5) also informs us of another disadvantage of 

 the new camera as compared with the natural one. In the latter, 

 the fact of the objects being at diflferent distances was of no im- 

 portance; in the former, however, the images of near objects 

 are more distant from the lens than are those of more remote 

 objects ; and since the plane of the screen cannot accommodate 

 all, it follows that if some images ai'C sharp, others cannot be so. 

 This inconvenience compels the photographer to have recourse 

 to many expedients (such as grouping of the objects, &c.), of 

 which some will be considered in the sequel. 



Again, the sharpest parts of the picture of a distant plane 

 object no longer fall in a plane, but on a spherical surface whose 

 radius is #;9=16i in., and whose concavity is turned towards 

 the lens. In consequence of this unavoidable circumstance, and 

 the many difficulties attendant upon photographing on curved 

 siu'faces, sharpness must be sacrificed the more the field of view 

 is increased. 



Above all other things, however, the restoration of achroma- 

 tism is the most important ; for the chromatic aberration dis- 

 appearing thereby, aperture and consequently illumination may 

 be increased, whilst at the same time the aberration arising from 

 diifraction will be proportionally diminished. As is well known, 

 this achromatism is obtained by a combination of crown- and 

 flint-glass lenses ; and the method which has long been employed 

 in telescopes not only leads to achromatism, but also diminishes 

 a new defect known as spherical aberration. The latter mani- 

 fests itself the more the greater the aperture, and is caused by the 

 spherical form given to the surfaces of the lenses, — a form which, 

 although most easily constructed, is not the one best adapted 

 for causing all the rays of a pencil to converge to the same point. 

 In ordinary telescopes the construction of the object-lens is 

 rarely based upon strict calculations as to the best curvatures 

 for obtaining the desired effects, the latter being determined ge- 

 nerally by trials. The crown-glass lens is biconvex, the flint- 

 glass one plano-concave ; and the two are placed together so as 



