94 
PRACTICAL PHOTO-MICROGRAPHY. 
as to their focal lengths there would be no trouble, for 
a iin. glass magnifies ten diameters on a plane ioin. 
behind its posterior principal focus, or, practically, 
behind its back combination. Then an ocular magni¬ 
fying three times in diameters would increase the 
magnification of the iin. glass at ioin. to 30 diameters ; 
and so on. But as neither objectives nor oculars are 
generally correctly designated, we require to use a stage 
micrometer, ruled to .01 and .001 in. or in fractions of a 
millimetre. The image of the divisions is projected 
upon the ground or plain glass and there measured ; 
the magnification being then easy to determine very 
closely. With the apochromatics and oculars of Zeiss 
a close measurement is obtained by dividing the 
camera-stretch from shoulder of ocular to sensitive 
plate x the number of the ocular by the focal length of 
the objective—all in millimetres. Thus, with a 3 mm. 
objective, a No. 3 projection ocular and 750 mm. from 
ocular-collar to plate, = 750 magnification. But 
alteration of tube length from 250 mm. throws this 
calculation out with objectives intended for the ioin. 
tube. 
Suitable tests for the high powers will be the 
secondary markings on some of the- finer diatoms ; 
“ dots ” on Surirella gemma, “ dots ” on Navicula 
rhomboides; the flagella on some of the coarser bac¬ 
teria, as Spirillum serpens, or minute bacteria them¬ 
selves, as B. Termo, or some Cocci. For these immer¬ 
sion glasses will be used, and the objectives carefully 
corrected by tube-length or collar. The condenser 
