106 PHOTOGRAPHING BACTERIA. 



require an amplification of five hundred to one thousand diameters ; 

 and as it is often desirable to make comparisons as to the dimen- 

 sions of microorganisms which resemble each other in form, it is 

 best to adopt a standard amplification. The writer has himself 

 adopted, and would recommend to others, a standard amplification 

 of one thousand diameters. This is about as high a magnifying 

 power as we can get with satisfactory definition, or as we require, 

 and it is a convenient number when measurements are made from 

 the photograph. The beginner, after having put his apparatus in 

 position, should focus the lines of a stage micrometer upon the 

 screen with the optical apparatus which he proposes to use ; then by 

 moving the screen forward or back as required, and carefully focus- 

 sing the lines, he will ascertain what is the position of the screen for 

 exactly one thousand diameters. If the stage micrometer is ruled 

 with lines which are one one-thousandth of an inch apart, it is evi- 

 dent that when projected upon the screen they should be one inch 

 apart to make the amplification one thousand diameters. But it 

 must be remembered that any change in the position of the optical 

 combination will change the amplification. If, therefore, the cover 

 correction of the objective is changed, or the position of the eyepiace 

 if one is used it will be necessary to again adjust the distance of 

 the screen. 



Apparatus required. A first-class immersion objective of one- 

 twelfth of an inch or higher power, a substantial stand which can be 

 placed in a horizontal position, and a camera which can be coupled 

 with the microscope tube, are the essential pieces of apparatus. If 

 sunlight is to be used a heliostat will also be required. 



The oil-immersion objectives of any good maker may be used, 

 but the apochromatic objectives and projection eyepieces of Carl 

 Zeiss, of Jena, are especially to be recommended. Indeed, those who 

 can afford it will do well to get Zeiss' complete apparatus, which 

 includes a stand having a mechanical stage and a camera mounted 

 upon a metal frame conveniently provided with focussing appliances, 

 etc. However, good work may be done with less expensive appa- 

 ratus. 



The stand should be substantial and well made, with a delicate, 

 fine adjustment. A mechanical stage is not essential, but is a great 

 convenience in finding and adjusting to the centre of the screen a 

 satisfactory field to photograph. The substage should be provided 

 with a good apochromatic condenser, and with appliances for moving 

 the condensing lens forward and back and for centring it, with dia- 

 phragms, etc. 



By the use of a high-power objective, like the one-eighteenth-inch 

 oil-immersion of Zeiss, the desired amplification may be obtained 



