130 



BACTERIA IN RELATION TO PLANT DISEASES. 



slow movement, a swing-out condenser (fig. 120), two substage iris diaphragms, and 

 various other conveniences. For example, the screw-heads, determining the cross 

 and sidewise movement of the section, are on the same axis and may be reached and 

 moved without changing the position of one's arm. 



The apochromatic objectives are the only ones recommended for bacteriological 

 work. They cost more than achromatic objectives, but expense is a minor con- 

 sideration. In hot, moist climates the older apochromatic objectives of Zeiss fre- 

 quently became clouded, but those made in recent years have given the writer no 

 trouble in the latitude of Washington. They yield sharp images even with high 

 eye-pieces. Of course, compensating oculars must be used with the apochromatic 

 objectives. It is de- ) 



sirable to have the 

 whole series of ob- 

 jectives and eye- 

 pieces, but if one is 

 limited for means, 

 very good work can 

 be done with two 

 objectives and three 

 oculars, viz, object- 

 ives 1 6 mm. and 3 



mm. 1.40 n. a., 



and 



compensating oculars 

 4, 6, and 12. 



The newer forms of the Abbe 

 camera furnished by Zeiss (fig. 121) 

 leave little to be desired in the way 

 of a drawing camera. 



PHOTOGRAPHY AND PHOTO- 

 MICROGRAPHY. 



For permanent records nothing 

 equals photography. It constitutes, 

 therefore, a very important special Flg- l20 '* 



part of laboratory work, and every student of pathology should make a knowledge 

 of this subject part of his education. Some of the following suggestions will be 

 useful to beginners. 



The Zeiss Double-Protar lenses, series Vila, are the best all round photographic 

 lenses made by that firm, and are excelled by none made by any firm. The back 

 or front lens is usually as good as the combination. Excellent photographic lenses 

 are also made by Voigtlaender and by Goerz. Zeiss photographic lenses may be 



*Fic. 120. Swing-out condenser and other substage arrangements on Zeiss photomicrographic 

 stand, No. ic. There is an iris diaphragm in D, and a second one in S, which is for use when the 

 condenser is thrown out as shown in this figure. D swings under when C is thrown into place. 

 W racks the entire substage up or down. 



