FUNDAMENTAL CONSIDERATIONS 
19 
and the top of the celloidin block must be constantly kept wet with 
80 per cent, alcohol. 
The object is placed in the object carrier and clamped in. By 
means of the graduated disk at the base of the feed mechanism the 
thickness, in terms of microns, is regulated after each stroke of the 
razor. 
Rotary Microtome .—When paraffin ribbons are desired, especially 
for the study of serial sections pf material, the rotary microtome 
surpasses by far the efficiency of the sliding type of instrument. The 
Spencer Rotary Microtome No. 820 is shown in Fig. 10 and its 
plan of construction illustrated in Fig. n. In this instrument the 
sliding part which carries the object clamp (SP) is carried up and 
down by the block (B). The feed mechanism consists of a rigid 
bearing, on which the feed block (FB) (of which the projection P is 
a part), is moved by the feed screw (FS). As this block travels to¬ 
ward the side on which the balance wheel (W) is located, the sliding 
part (SP) is forced forward towards the knife one-half as much. The 
polished surface set against the point (P) is arranged at the proper 
angle to accomplish this end. The screw, cut with two threads to 
the millimeter, is revolved by a ratchet feed wheel with 250 teeth. 
Each tooth represents a forward movement of the object of one mi¬ 
cron. The feed can be set for sections from 1 micron to 60 microns 
thick, by turning the button at the back of the case until the number, 
representing the desired thickness, appears opposite the indicator at 
the small opening in the side of the case near the balance wheel. 
The total excursion of the feed is 37 mm. This allows a sufficient 
range for cutting a complete series of sections of a large object with¬ 
out the necessity of a break due to resetting the knife and feed me¬ 
chanism. The object, after being placed in the object clamp, may 
be oriented to any desired angle. The clamp is held at its upper 
limit for orienting or trimming the block by pushing in the pin (F.) 
The whole knife support may readily be adjusted to and from the 
object, and is readily clamped in any location by a lever connected 
with an eccentric cam. The knife is fastened by two clamps and 
may be turned to any desired angle. The clamps can also be moved 
toward each other to bring them as near to the ribbon as desired to 
gain additional rigidity. The groove in the balance wheel is de- 
