228 PEACTICAL PHOTO-MICEOGKAPHY 



bright ; owing to the fact that it transmits a good deal of the 

 actinic portion of the spectrum it may only increase exposure 

 from four to five times. On the other hand, with deep screens 

 of a yellow-green variety, an increase of forty to fifty times is 

 by no means unusual. 



In general it may be said that a systematic series of exposures 

 made on objects presenting no colour difficulties, with the source 

 of light generally in use and with a given plate, will afford 

 more information especially if records are kept of the 

 conditions under which the photographs are taken than any 

 amount of instruction. 



