ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 607 



growth of the Bacillus coli group, the streptococci, B. suMilis, B. proteus, 

 and Jluorescens, is so luxuriant that in comparison normal broth may be 

 said to give a scanty growth. 



(3) Cutting, including- Embedding- and Microtomes. 



Spencer Automatic Laboratory Microtome.* — This apparatus, 

 numbered 880 in the maker's catalogue, is shown in fig. 41. In this 

 microtome the main supporting frame has a heavy clamp at the back by 

 which it is securely fastened to the laboratory table. As will be seen 

 from the figure, the upper part of this frame forms a support to which 

 the two laterally swinging arms are attached by steel pivot screws with 

 check nuts. The knife-carrier is held by these swinging arms at their 

 outer ends, attached thereto by similar pivot screws with check nuts, 

 and in order to give the proper movement, relieved from any pressure 

 or strain, a detachable flexible lever handle is attached to the axis 

 fastened to the longer arm on which the arm swings. The extreme 

 ends "of the knife rest in the holder, and, as the lever moves the swing- 

 ing arms, the blade describes the flattened curve, corresponding to the 

 double movement in free-hand sectioning. By this manner of holding 

 the knife by arms, which are not parallel, the entire length of its cutting 

 edge is utilized, insuring uniform wear and permitting the cutting of 

 larger sections than has heretofore been possible, except by using a very 

 much larger blade. The swinging arms and knife-holder are sufficiently 

 rigid to avoid any deflection of the knife in its movements ; thereby 

 assuring uniformity of thickness in all the sections. This peculiar 

 motion of the knife makes this microtome especially desirable for cutting 

 frozen sections. Sections may be cut, stained and mounted in one and 

 one-half minutes from the time the tissue is placed on the freezing 

 plate. This microtome does excellent work with celloidin also. The 

 movement of the knife is not so well suited to paraffin work, but it does 

 very well with this medium. 



Another important advantage in this method of construction is its 

 convenience to the user. In most microtomes with sliding parts, these 

 require frequent lubrication with oil, and in consequence are liable to 

 be clogged with dirt, requiring frequent cleaning, and also to become 

 loo.-e and shaky. In this microtome, lubrication is not required, and 

 in its construction every joint is provided with pivot screws and check 

 nuts, and by means of a steel pin sent for the purpose the owner may 

 take the instrument apart and adjust or clean it as occasion may require. 

 The object-clamp for paraffin or celloidin blocks has a pin or round 

 shank which may be fastened by a clamp screw in a vertically movable 

 socket which is supported by two vertically swinging arms attached at 

 the back to the main frame by hardened steel pivot screws. Similar 

 pivot screws at the front hold the socket in the swinging arms; thus 

 providing for its vertical movement the same parallelogram principle as 

 is applied to the knife. By this means, a steady movement is secured 

 and the top of the object always remains in the same horizontal position ; 



* Catalogue, Spencer Lens Company, p. 113. 



