STEREO-PHOTOMICROGRAPHS. 463 



people's resources, but it enables some very rapid exposures to be 

 made. Using an open arc, with a current of 50 amperes, I have 

 made a fully exposed negative of some aecidia on a nettle leaf 

 x 67 with an exposure of 2 seconds. In spite of a deep cooling- 

 chamber, one stands a great chance of burning a specimen with 

 such a powerful light, unless great care is used. It is a great 

 help to have a powerful arc when dealing with autochromes. 



I should like to say a few words about autochromes. It is a 

 matter of regret that these wonderful plates do not appear to be 

 suitable for stereoscopic work. The trouble is not so much in the 

 length of exposure as in their patchiness. It is impossible, in 

 practice, to mix the starch-grains, of which the tri-colour filter 

 is composed, in accordance with theoretical requirements. A 

 number of grains of the same colour will persistently adhere to 

 each other, these forming distinct colour blotches, which float in 

 space over the picture proper when seen in the stereoscope. 



As photomicrographic work is usually done with an artificial 

 source of light, the usual autochrome filter is unsuitable. A 

 special filter is made for the electric arc, which I have not used 

 with any great success. It is doubtless correct for a certain 

 composition of the carbons ; but these are continually varying, 

 creating small differences in the colour composition of the light to 

 which these plates are exceedingly sensitive. 



A very good method of working is to convert the light from a 

 Nernst lamp to theoretical daylight by a special blue filter made 

 for the purpose, and then use the usual daylight filter. This 

 necessitates very long exposures ; but the results are well worth 

 the extra trouble involved, as the colour rendering is as accurate 

 as one could possibly wish. 



Description of Plates. 

 Plate 37. 



a. Lamp casing containing hand-feed arc lamp. 



b. Lens to parallelise rays from arc. 



c. Water-cooling chamber. 



d. Long-focus lens, converging the parallel rays, after reflecting 



from mirror h, on the object o. 



e. Short-focus lens. 



