ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 299 



be cut. This second carrier moves on an inclined surface, so as to 

 raise the specimen as required. 



This, with a few modifications, is the general character of all 

 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 all show 

 more or less imperfect results, owing to the fact that it is impossible 

 to obtain sufl&ciently exact plane surfaces. The inconveniences 

 appear in small irregularities of the movement of the carriers, and 

 the consequent impossibility of making sections as thin as with an 

 experienced hand. 



This induced Prof. 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 planes, even if they are not 

 absolutely geometrical planes, or the angle which they include is not 

 everywhere the same. Such a carrier will always take exactly the 

 same course ; and, in consequence, a knife attached to it will cut a 

 series of perfectly parallel sections through an object which is suc- 

 cessively raised to a higher plane after each cut. The working of the 

 instrument will therefore be far superior to any microtome with 

 large sliding surfaces which nowhere exactly fit the sliding surfaces 

 of the stand. This indicates the desirability of constructing the 

 carrier for the object on five points also. 



The construction resulting from these principles is simple and 

 practical, but it is necessary to take into consideration the centres of 

 gravity of the different sliding bodies. This, however, complicates 

 the matter but very little. "We replace the two sliding surfaces of 

 each carrier by five slightly prominent points, and they will then move 

 with exactness on any combination of two planes, not differing too 

 much from geometrically plane surfaces. One condition only must 

 be fulfilled, namely, that the five points are so chosen as to support 

 steadily the centre of gravity of the carriers, including their acces- 

 sory parts — namely the knife and object. Fig. 52 gives a more 

 precise idea of the details of construction. 



In the figure the lower surfaces of the carrier a, which supports 

 the knife show three prominences, which gives the geometrical projec- 

 tion of the five points. Within the limits of the figure these points 

 could not be drawn exactly as they are in the instrument itself. In 

 reality, they appear only as small prominences upon three narrow 

 ridges on the sliding surfaces of the stand. This arrangement was 

 desirable to facilitate the action of the oil with which the sliding 

 surfaces are to be covered. Two of the ridges form together parts of 

 the oblique plane, and the third corresponds to the vertical sliding 

 plane. The same arrangement is found in the carrier h, which 

 supports the clamp in which the object is placed. 



By this mode of construction the carriers will move gently and 

 regularly even if the sliding surfaces of the stand are not perfect 



