116 THE MICROSCOPICAL NEWS. 
cardboard, having its centre coincident with the axis of the fine 
adjustment screw, is placed behind the pointer, and marked in its 
circumference with degrees. The split tube allows of the entire 
revolution of the adjustment screw, whilst the pointer acts as an 
index through go° of arc, and records with exactness any slight 
alteration in focus. The necessity for this will be hereafter ex- 
plained. 
As correct focussing is a most essential point in photomicro- 
graphy, the finest ground glass is inadequate for viewing the image 
for final adjustment. ‘The screen is therefore removed, and a lens 
of short focus, mounted in the middle of a light lath, two inches 
wide and fourteen inches long, is so adjusted in a short tube that 
its focus is coincident with the plane of the sensitive film when the 
Fig. 36. 
lath is held against the back of the camera. This operates as an 
eye-piece, and the adjusting rod is turned till the picture appears 
in sharp outline. 
At the side of the base-board a scale of inches is marked, 
measuring from the object carrier, for recording the length of focus 
employed. Scales of diameter under the respective object-glasses 
can also be added, showing at a glance the magnification obtained. 
Thus, at thirty-six inches, it is definitely seen on this instrument 
that the two-inch objective gives twenty-one diameters, 
The chief difficulties to be surmounted by the novice occur 
under the following heads :—1. Non-coincidence of actinic and 
visual foci of lenses. 2. Even illumination. 3. Exposure. 4. 
Selection of suitable objects for photography. 
Actinic AND VisuaL Foct.—In photographic lenses by good 
