24 BACTERIA IN RELATION TO PLANT DISEASES. 



Good pliotoinicrograplis should be secured if possible. Koch's first photo- 

 micrographs were of various eulargenieuts. He afterwards recommended X i,ooo 

 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 achronratic ones. For very small magnifica- 

 tions the writer has foirnd 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. 



A 



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 

 , / consequence. Direct sunlight is the best light, but 



(:y^^^ 2 y the light of the open sky may be used (with full open 

 Jp J? "i, U \\ diaphragm) if one is willing to make 5 to 20 minute 

 M li fl exposures. Electric light is often used by those who live 



^ ^ in cloudy regions or who occupy rooms not exposed to 



PI 21 + the sun, but the writer has had no experience with it. 



Very good pictures also may be made b}- 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- c=:-p j^j"^ 

 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 Car1)utt or by Ives. 

 In using a horizontal apparatus, such as that '^' ^^- 



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



*Fji-.. 21) —Hollow-ground slide with cover-glnsb bearing hanging drop for examination under 

 the nii'CroS'Cope. 



tFic. 2T — Involution forms ni Bacillus '/•ai:/u'//'/H7H.j from extremely ropy potato brobli. Drawn 

 free hand, / 1,000 circa. l\'I;uiy as large as 8 by 2 micra and otlicrs larger. Nov., 1894. 



jFio. 2J.— Y -sliapcd (dicbolomously branched) bodies from the root-tubercles of clover (Tri- 

 folium). From a photomicrograph by fhe author, made from a slide furni.sihedby Dr. Geo. T. Moore. 

 y 1,500. 



