38o 



NATURL 



[November i8, 1920 



microscope is of sufficient accuracy (the searcher 

 eye-piece is not used in this case except to con- 

 lirm the accuracy of the process). The focussinfj 

 havinff been performed, the searcher eye-piece is 

 removed, the camera phiced in position, and 

 the exposure made. The imafje is projected for 

 a certain distance, so that it is in focus at the 

 plane of the plate with a known length of can-.era. 

 The exposures required are as short as two 



quantity of gelatine, but with the maximum quan- 

 tity of sensitive silver salts that the gelatine can 

 hold together. Such a plate has been prepared 

 by the Kodak Co., and has proved satis- 

 factory. Plates as prepared by Schumann for 

 work in the far ultra-violet have also been experi- 

 mented with, but for various reasons have not 

 proved so satisfactory. The resulting negatives 

 are at first glance somewhat disappointing if 



jrr 







^ 



Fig. 2. — Saccharomycts I'listorrttttHs {'ic^^i). X 1700. 

 Illuminated by mrans of concentric diirk-ground illuminator. 



Fit;. 3. — S.iccharoniyccs Pastomiiius iy^SiSi), 

 lliuminated by ultra-violet light. 



- ». 



t IG. 4- — liacitlus anthracis. X isctj. 

 Dark-ground illumination. 



Fig. 



-Bacillus anthracis. 

 Ultra-violet light. . 



X 1700. 



seconds under favourable conditions, even at high 

 magnifications. 



There was considerable difficulty in obtaining a 

 suitable photographic plate, as one was required 

 of fine grain and with the smallest possible quan- 

 tity of gelatine on its surface. Gelatine is itself 

 opaque to ultra-violet light, so that the photo- 

 graphic action is confined to the surface of the 

 gelatine, little or no penetration in depth taking 

 place as with ordinary light. The result is that 

 plates must be prepared with the smallest possible 



NO. 2664, VOL. 106] 



judged by ordinary photographic standards. They 

 are always thin and lacking in violent contrasts, 

 owing to the superficial action of the light, but 

 the detail and fineness of lines due to the shorter 

 wave-lengths used are evident to anyone having 

 any knowledge of photomicrography. Whether 

 the utmost resolution that theory demands can 

 be achieved is at present unproved, not because 

 the method has failed, but because of the difficulty 

 of finding an object that can be regarded as a 

 satisfactory test. 



