144 THE PRODUCTION OP POSITIVES, ETC. 



unscrewed from the camera; the cell containing 

 the back combination removed ; the lenses are taken 

 ont of it; the ring separating them is next removed 

 and put back in the cell, and the lenses replaced in 

 their original order but in contact with each other. 

 By this means the definition is considerably im- 

 proved, and the consequent decrease of focal depth 

 prevents the images of objects, situated beyond the 

 film of the negative, being impressed on the sensitive 

 surface. 



The modified objective is fitted to the camera and 

 levelled at the negative. The latter should be 

 placed in a window having a northern aspect and an 

 uninterrupted view of the sky. To obtain this it is 

 sometimes necessary to tilt the camera upwards, 

 great care is then required to keep the surface of the 

 ground glass parallel with the negative, and both 

 perpendicular to the optic axis of the objective. 



The most accurate focus, with the full aperture of 

 the objective is obtained; a stop of sufficient size in- 

 troduced, and the camera covered with a black cloth. 

 A great improvement will be effected, and the en- 

 trance of all extraneous light prevented, by pinning 

 the black cloth round the negative, so as to form a 

 sleeve between it and the camera ; it must however 

 be left unfastened at the bottom to allow of the re- 

 moval of the cap on exposure. 



Before describing the remaining manipulations it 

 is necessary to remind the operator, that the aper- 

 ture of all stops or diaphragms are best named in 

 terms expressing their relation to the focal length 



