1883. ] Microscopy. 993 
sliding microtomes ; but hitherto the carriers were constructed to 
slide with two even surfaces between two even planes of the 
stand, which intersect at a given angle, with the consequence that 
show more or less imperfect results, owing to the fact that it 
is impossible to obtain sufficiently exact plane surfaces. The in- 
conveniences appear in small, scarcely perceptible irregularities 
€ movement of the carriers, and the consequent impossibil- 
ity of making sections as thin as with an experienced hand. 
This induced Professor Thoma to enter upon a consideration 
of the geometrical and mechanical difficulties to be surmounted. 
The question to be solved was, how many points at least of a 
body sliding between two planes must touch the latter for this 
body to be perfectly steady in its position. It will be found that 
five points are sufficient, and that a carrier on five points, between 
two plane surfaces, will slide without difficulty between these 
TTT TAMATA REED SEN 
Fic, 1.—Thoma’s microtome. a, carrier for the knife; 4, carrier for the object; 
*, micrometer-screw for fine adjustment. 
planes, not differing too much from geometrically plane surfaces. 
€ condition only must be fulfilled, namely, that the five points 
are so chosen as to support steadily the center of gravity of the 
