Mr. Cundell on the practice of the Calotgpe Process. 323 



camera obscura, from its property of producing a compara- 

 tively fiat and focal field throughout the picture, when the 

 picture is received upon a plane surface (§ 9). Without under- 

 valuing the advantage of corrected aberration, it may well be 

 doubted whether you do not lose more than you gain by the 

 plano-convex figure, even though achromatic, from the im- 

 possibility of bringing its picture, when of any extent, to a 

 tolerable focus. Achromacy is no doubt desirable; but in calo- 

 type, where the image is not to be magnified, it is by no means 

 indispensable, as any one may prove who fairly tries the ex- 

 periment; and the expense of a really achromatic lens of an 

 adequate aperture must put it in a great degree out of the 

 question. Perhaps the best substitute for it would be a lens 

 of blue glass, which would transmit nearly the whole of the 

 chemical rays to a common focus. But of whatever figure the 

 lens may be, and of whatever colour, it will not be unimportant 

 that the focus be considerably longer than that commonly used. 



7. In order that a picture in perspective may be seen with 

 truth and satisfaction, it is necessary that it be seen from a 

 particular point of view, in which the eye has the same rela- 

 tion to the picture that it would have to the object represented. 

 The picture must subtend at the eye the same angle as the 

 object ; and unless it do, it will always look more or less dis- 

 torted and unnatural. The principle is well illustrated in 

 the diorama, the illusion and the charm of which depend in 

 no small degree upon the placing of the spectator at the proper 

 height and distance; but the principle applies to all pictures 

 in perspective, and to camera pictures in particular, which are 

 wonderfully improved when placed at the proper distance from 

 the eye. Calotype pictures are not intended to be looked at, 

 and are seldom viewed, at a shorter distance than twelve inches; 

 and in order that such a picture viewed at that distance may 

 be seen in true perspective, the lens of the camera must be of 

 twelve inches focus. In portraiture the effect may be less ob- 

 vious than in architecture, or in general subjects ; but there 

 can be no doubt that a portrait taken by a lens of six inches 

 focus, viewed at the distance of twelve inches, would lose a 

 great part of any truth or likeness it might really possess. 



8. For these reasons the lens oughtnot perhaps to be less than 

 twelve inches focus; and, if mounted in the manner shown in the 

 subjoined drawing, it will be found to be generally convenient. 



There is no novelty in this construction, unless perhaps in 

 the introduction of the diaphragms A B and C D, and in the 

 elongation of the mouthpiece; both of which are useful in pro- 

 ping out and admitting the light, is precisely the part which (so far as I am 

 aware) has been entirely overlooked in practice, and in every popular trea- 

 tise on the subject. 



Y2 



