138 



BACTERIA IN RELATION TO PLANT DISEASES. 



before they enter the objective; otherwise, if the focus of the condensing lens should 

 accidentally coincide with the balsam mount of the lenses for a few minutes, it may 

 be softened and the objective ruined. Pure water is as good for this purpose as alum 

 water, which was formerly much recommended. It removes more than 50 per cent 

 of the heat rays. 



The writer uses a Zeiss 3-inch mirror with micrometer-screws for throwing the 

 sun's rays. This serves quite as well as the more expensive heliostat, if one can 



Fig. 126.* 



work quickly. The order ot procedure is to obtain the proper focus and see that it 

 " holds ; " the plate holder is then introduced and opened, and consequently the bel- 

 lows must be light-tight ; last of all, the sunlight is accurately re-centered and the 

 shutter snapped. The photomicrograph should be made with light from the central 



*Fic. 126. Fragment of a green leaf bearing black spots. Enlarged 6*/ 2 times with a Zeiss 

 Planar lens and photographed on a Seed's 27-X plate. Introduced for comparison with fig. 127. 

 Notice that although stopped down considerably, part of the leaf is out of focus. 



