240 PHOTOGRAPHY WITH COLOR SCREENS [Ch. VII 



Fortunately the manufacturers furnish the information concerning 

 their plates and the color filters, so that labor is spared the individual 

 worker. It might be worth while for him to check up the color screens 

 occasionally to make sure that they have not deteriorated. 



§ 372. Time of exposure for photo-micrographs. — This varies 

 from the fraction of a second to several minutes, depending on four 

 factors: 



(i) The nature and intensity of the light. 



(2) The magnification of the microscope. The higher the mag- 

 nification the longer must be the exposure. 



(3) The transparency of the specimen. The more transparent 

 the shorter the exposures. 



(4) The thicker or deeper the color of the ray filter the longer must 

 be the exposure. 



Light for Photo-micrography 



§ 373. Daylight. — This has served for some of the best photo- 

 graphs which have ever been made. If it is not available, artificial 

 daylight obtained by using daylight glass forms a very good substi- 

 tute (§ 97). 



§ 374. Artificial lights. — As compared with daylight all ordinary 

 forms of artificial light have a great excess in the red end of the spec- 

 trum (see fig. 36, comparing the mazda and daylight). This excess 

 in the red end has the advantage that it partly compensates for the 

 excessive sensitiveness of the photographic plate for violet-blue 

 light. For many objects a kerosene lamp is excellent for photograph- 

 ing by, as it serves for both light and color screen. 



§ 375. Mutual adaptation of color screen and light. — As the 

 color screen is for a very definite purpose in absorbing certain parts 

 of the light it follows that the character of the light and that of the 

 color screen must be mutually adapted. For example it is self-evi- 

 dent that the same color screen for a given preparation would not serve 

 for both daylight and the light from a mazda lamp (see fig. 36). So 

 also the same color screen would not be successful if used both for 

 the mazda light and for the light of a kerosene flame. 



