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. hey yield sharp images even with high 
eye-pieces. Of course, compensating oculars must be used with the apochromatic 
objectives. It: is de- g 
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 16 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, Mts 
therefore, a very important special i oe 
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 VIIa, 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. tc. 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. 
