EXPOSURE 



173 



Effect of Magnification. Exposure varies as (M) 2 : 



Magnification. 



10 



25 



50 



100 



250 



500 



1000 



Exposure Factor. 



iTTO- 

 TV 



i 

 1 



6 



25 

 100 



Effect of Light Source. This factor varies very greatly 

 with the colour sensitiveness of the plate used. The 

 figures given are based on those required by Wratten's 

 special photomicrographic panchromatic plate " M." 

 Those in the columns headed " Orthochromatic " and 

 " Ordinary " refer to plates of the same speed in daylight, 

 namely, 100 Watkins or 68 H and D. The " Allochrome " 

 plate would require the orthochromatic factors to be 

 multiplied by ^, as its speed in daylight is 2J times as 

 great as that of the "M" plate, but if orthochromatic 

 plates of other make are used, the simple ratio of their 

 speed numbers in daylight will not necessarily be correct, 

 as their greater or less sensitiveness to yellow, etc., is of 

 much greater importance, so that c the factor for each must 

 be determined by experiment. The column headed 





