156 THE PRODUCTION OF POSITIVES. 



binations, which in addition to their costliness 

 possess inherent faults that cannot be corrected. 



If we desire a copy the same size or smaller than 

 the original ; the brass tube containing the lenses is 

 unscrewed from the camera ; the cell containing the 

 back combination removed ; the lenses are taken out 

 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 improved, 

 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. 

 When the most accurate focus, with the full aperture 

 of the objective is obtained ; a stop of sufficient size 

 is introduced, and the camera covered with a black 

 cloth. A great improvement will be effected, and 

 the entrance 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 removal of the cap on exposure. 



Before describing the remaining manipulations it 



