24 BACTERIA IN RELATION TO PLANT DISEASES. 
Good photomicrographs should be secured if possible. Koch’s first photo- 
micrographs were of various enlargements. He afterwards recommended X 1,000 
as the standard magnification, but x 1,500 and x 2,000 are also convenient sizes and 
occasionally Xx 500 is better than X 1,000. Most important is it that the exact mag- 
nification should always be indicated. ‘The Zeiss apochromatic objectives are much 
better for photographic work than the achromatic ones. For very small magnifica- 
tions the writer has found the old Zeiss 35 mm. and 70 mm. very useful. For the 
same purpose the newer Zeiss planars, series Ia Nos. 1-5 (fig. 122) are admirable. 
These have sharp definition and a very flat field, but not much depth of focus. With 
them objects several centimeters in diameter may be satisfactorily photographed with 
magnifications from 2 or 3 diameters to 50 or more. ‘The writer obtains as sharp a 
focus as possible with wide-open diaphragm and then stops down about two-thirds. 
= x] 
Fig. 20.* 
One of the best simple photomicrographic outfits is the Zeiss upright camera 
(fig. 24). All apparatus is to be rejected which requires the microscope to rest on 
the same platform as the camera. It should rest on the table independent ot 
the camera, unless a weak light is used and the exposures 
\ > ) are very long, in which case a slight jarring is of no great 
) ia consequence. Direct sunlight is the best light, but 
p () Go, the light of the open sky may be used (with full open 
(A as j f diaphragm) if one is willing to make 5 to 20 minute 
( exposures. Electric light is often used by those who live 
) in cloudy regions or who occupy rooms not exposed to 
the sun, but the writer has had no experience with it. 
Very good pictures also may be made by gaslight if the 
Welsbach burner is used. Ordinary lamp light (kerosene) is too yellow and not 
sufficiently intense. Photographs can be made 
with a kerosene light, but the time and trouble Se (| 
involved make it scarcely worth while to 
consider this source of light. ‘The writer has aR KR. 
obtained the best results by using direct sun- ae 
light and slow isochromatic plates behind Zett- Oe AS LR 
now’s light filter. Of course, with upright 
cameras a dry light-filter must be used, such as S 
the yellow one devised by Carbutt or by Ives. 
In using a horizontal apparatus, such as that 
shown in plate 5, the-szze gua non is to get it properly leveled up and to keep it so. 
Fig. 22. 
*Fic, 20.—Hollow-ground slide with cover-glass bearing hanging drop for examination under 
the microscope. 
{Fic. 21.—Involution forms of Bacillus tracheiphilus from extremely ropy potato broth. Drawn 
free hand, X 1,000 circa. Many as large as 8 by 2 micra and others larger. Nov., 1894. 
{Fic. 22— Y -shaped (dichotomously branched) bodies from the root-tubercles of clover (Tri- 
folium). From a photomicrograph by the author, made from a slide furnished by Dr. Geo. T. Moore. 
X 1,500. 
