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 microérganisms 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 eyepi2ce 
—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, ete. 
By the use of a high-power objective, like the one-eighteenth-inch 
oil-immersion of Zeiss, the desired amplification may be obtained 
