THE BOOK OF THE LANTERN. 221 



on A. It will be readily seen that the apparatus may be 

 rendered more effective by the attachment of a screw, or 

 fine adjustment for more accurate focussing ; but with the 

 lower powers this would not be needed. It is an addition, 

 however, which the makers will supply when required. 



All the objectives made by Messrs. Swift, as well 

 as those by a few other makers, are corrected for 

 photography, but in those of older date the visual focus 

 does not agree with the chemical focus. As a rule, no 

 difficulty will be found with the higher powers, and with 

 the others a few trials will soon show what allowance in 

 focussing must be made. As a rough guide to what must 

 be done to correct this fault, which is rendered evident 

 by a sharply -focussed image appearing blurred in the 

 negative, proceed as follows : First focus the image as 

 sharply as possible, and then cause the objective to 

 approach the object until the latter seems to be sur- 

 rounded by a reddish light; now take the photograph, 

 and it will be sharply defined, although the image did not 

 look so on the focussing screen. 



The ferrous-oxalate method of development, described on 

 a former page, is very suitable for negatives taken for 

 photo-rnicrographs. These negatives are then made to 

 furnish lantern slides in the manner detailed in a former 

 chapter. 



