124 



PRACTICAL PHOTO-MICROGRAPHY. 



to meet certain peculiarities of negative. In varying the 

 distances, however, we must not overlook the law regarding 

 the intensity of light, viz : that the intensity varies inversely 

 as the squares of the distances between radiant and recipient ; 

 in other words that halving the distance is equivalent to 

 quadrupling the exposure. If the correct exposure at 18 inches 

 be twenty seconds, at 30 about 55 seconds will be required, as 

 18 2 : 302 : : 20 : 55$. 



If magnesium ribbon be used it will be found convenient to 

 burn each time a definite length, say one inch, and to vary the 

 distance of the printing frame from the burning wire. A 

 number of prints may be exposed at one operation with 

 magnesium wire by arranging the frames in a circle, or at 

 various parts of several circles, either imaginary or actually 

 traced on the table. 



FIG. 31. 



By such an arrangement as shown at figure 31 we may 

 evidently expose with one length of wire or ribbon, burned at 

 " X," eight negatives of three grades of density at one time. 



" Bromide paper," as it is usually called by photographers, 

 is a good quality of paper coated with an emulsion of gelatine- 

 bromide of silver, a considerable proportion of chloride 



