EDUCATIONAL VALUE OF PHOTOMICROGRAPHY. 149 



light, whether ordinary or polarized; the kind of stain used upon the 

 slide, if any; the magnifying power employed, and the rapidity of 

 plate, all combine to determine the limiting values for exposure. There 

 is scarcely opportunity in this paper to enter into discussion of these 



Fig. 5. Crystal of Chloritoid in 

 Quartz, x 110 Diameters. Polar- 

 ized Light. 



Fig. 6. Band of Quartz Crossed by 

 Mica, x 100 Diameters. Polarized Light. 



factors in detail. Jn general, however, it may be said that the blues 

 transmit more actinic light than they appear to do; and so there is 

 danger of over-exposure, while with the dark yellows and reds it is quite 

 the reverse. Other conditions remaining the same the exposure is 

 directly proportional to magnifying power, and since frequent changes 

 in objectives and oculars are necessary to obtain the desired magnifica- 



Fii;. 7 Section of Ikon. X 60 Diameters. 



tion of different objects, the calculation of the exposure in terms of 

 magnification expressed in diameters simplifies the work considerably. 

 With living organisms that are given plenty of space to move about, in 

 order to be photographed under conditions favorable to them, the 

 exposure must necessarily be very short, and of course only moderately 

 high powers can be used, and the intensity of the light must be very 



