83 General Notes. 
cm. As the distance 6 c is constant, 10 cm., c ;2 must be 30 cm., 
and may be easily found on the ruled rod that supports the camera. 
Next a convex eye-glass of 10 cm. focal distance is inserted at d, 
and the upper end of the sliding ring to which the stage is attached 
brought within 10 cm. of the lower edge of the ring to which the 
camera is fastened. 
The amount of light is regulated by means of smoked glasses in- 
serted above the convex glass at d. 
If the eye of the observer is myopic, it is necessary to insert at/ 
an eye-glass for correction. A myopic person will often find it con- 
venient to use a glass a little stronger than is required in looking at 
distant objects. 
Finally, in all cases, except where a majinifying power of from 
1 to 2 times is used, a diopter (g) must be placed above /. In 
using the high magnifying powers the focal points of both systems 
do not exactly coincide, so that a parallactic displacement of the 
images is produced, if the diopter is left out. This is a defect of 
all cameras and is us n ally corrected by the use of small prisms, 
while here the same object is equally well, and at the same time 
more conveniently, accomplished by the diopter. 
The magnifying power is equal to the ratios of the distances, bcz 
and ^ = 40 : 10 = 4 : 1. 
In using the camera, it must stand before the observer, as in fig. 2, 
with the drawing on the right and the diopter and[Qbject on thejeft. 
