24 BACTERIA IN RELATION TO PLANT DISEASES. 



Good photomicrographs should be seciired 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. 15 (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. 



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 

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



(1 consequence. Direct sunlight is the best light, but 



J} \i ^~~ A 8 t ^ ie ^&ht f tne P en s ky ma y be used (with full open 

 " <? 9 (J diaphragm) if one is willing to make 5 to 20 minute 

 (j C\ exposures. Electric light is often used by those who live 

 \l 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 

 sufficiently 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. 

 In using a horizontal apparatus, such as that 

 shown in plate 5, the sine qua non is to get it properly leveled up and to keep it so. 



*Fic. 20. Hollow-ground slide with cover-glass bearing hanging drop for examination under 

 the microscope. 



fFic. 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. 



JPiG. 22. Y -shaped (dichotomously branched) bodies from the root-tubercles of clover (Tri- 

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

 X 1,500. 



