EXPOSURE 171 



to the strip of best printing density is the correct ex- 

 posure for the subject, plate, and light used. Trial 

 exposures should always be in geometrical progression, 

 and not with equal intervals between them. 



Record of Exposures. Experience enables one to 

 judge with very fair accuracy the exposure required under 

 any given conditions, but particulars of all photo- 

 micrographs taken should be entered in a notebook at 

 the time they are made, with details of subject, light, 

 lenses, screens, magnification, and exposure. In par- 

 ticular, making a note of any peculiarities in the subject 

 or errors made. A systematic entry of such details as 

 those indicated in the accompanying excerpt from an 

 exposure notebook is of the greatest assistance even to 

 an experienced worker, who can turn back at any time, 

 when in doubt, for the particulars of work previously 

 done of a similar nature to that for which he requires 

 guidance. The best magnification to give, the best 

 combination of lenses or aperture of condenser to use, the 

 most suitable screen to employ, can be more easily 

 decided upon when the results obtained in previous 

 experiments are recalled. 



Every negative is numbered when made either by 

 writing on the glass side with a diamond, or on a corner 

 of the film side in pencil or ink. If pencil is used the 

 figure should be written on the plate before development, 

 as it takes the writing more easily then. 



Factors Influencing Exposure. The most important 

 factors which govern exposure are the following : 



1. Light source Intensity and colour. 



2. Colour screen . /Spectral transmission and 



(^ transparency. 



3. N.A. of objective. . . . Exposure varies as /T^ ,TT 2 * 



