PHOTOGRAPHY AND PHOTOMICROGRAPHY. 



135 



placing a circular black disc centrally some distance behind the plate to be photo- 

 graphed, using for illumination the diffused light which comes in around this disc. 

 The result is a very sharp contrast, i. <?., white colonies on a black background 

 (Bull. Torrey Bot. Club, 1893, Vol. XX, p. 357). 



In photographing test-tube cultures the chief trouble is the great number of 

 confusing high-lights due to the curved surface of the glass. From an artistic 

 standpoint these are to be desired, but inasmuch as they are sometimes liable to be 

 mistaken for bacterial growths the naturalist desires to eliminate them. This may 



Fig. 125.* 



be done in several ways. One of the best wa}'s is to photograph the tubes through 

 a thin sheet of distilled water. For this purpose jars of clear white glass are 

 necessary. These should be about 5^ wide X 5f deep X i ^ inches thick (inside 

 measure), with parallel walls and a flat bottom. Such jars may be obtained of Emil 

 Greiner. Only those without flaws or wavy lines should be accepted. Better jars 

 with perfectly parallel flint-glass walls may be had from Carl Zeiss. Good results 



♦Fig. 125. — Showing method of holding Petri-dish poured plates for photographing by trans- 

 mitted light with camera shown in plate 17. The dish Is held in place by two stretched rubber 

 bands, exactly as if it were a negative to be used for making lantern slides. For manner of sus- 

 pension, ». e., relation to the camera, see plate 17. The organism is a 48-hour agar culture of van 

 Hall's Ps. syringae II, grown at 24° to 27° C. 



