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
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| Fig. 5. Crystal of Chloritoid in Quartz, 110 Diameters. Polarized Light. | Fig. 6. Band of Quartz Crossed by Mica, 100 Diameters. Polarized Light. | |
factors in detail. In 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 magnification
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
