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 X 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 la 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. 



I ~ *l 



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 ol 



the camera, unless a weak light is used and the exposures 

 are very long, in which case a slight jarring is of no great 

 consequence. Direct sunlight is the best light, but 

 the light of the open sky may be used (with full open 

 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 

 F . 2| . 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 



sufficient!}' intense. Photographs can be made 



with a kerosene light, but the time and trouble 



involved make it scarcely worth while to 



consider this source of light. The writer has 



obtained the best results by using direct sun- 



light and slow isochromatic plates behind Zett- 



now's light filter. Of course, with upright 



cameras a dry light-filter must be used, such as 



the yellow one devised by Carbutt or by Ives. <iS 



In using a horizontal apparatus, such as that 



shown in plate 5, the x/'/a' </na noil is to get it properly leveled up and to keep it so. 



I'n.. _>i> ---IIiillow-Kfouinl slide with cover-glass bearing hanging drop for examination under 

 the microscope. 



tFic. 21. Involution forms of Bacillus tracheiphilus from extremely ropy potato broth. Drawn 

 in i hand, X 1,000 circa. Many as large as 8 by 2 micra and others larger. Nov., 1894. 



tFif.. 22. Y -sliapcd (dichotomonsly branched) bodies from the root-tubercles of clover (Tri- 

 folium). From a photomicrograph by tlie author, made from a slide furnished by Dr. Geo. T. Moore. 

 X 1,500. 



