ZOOLOGT AND BOTANY, MICROSCOPY, ETC. 



539 



to show that it affords the most convenient and surest means of raising 

 the object, as applied in the microtome which he has devised. Errors 

 in the construction of the screw are not cumulative, and will not 

 amount to anything that is appreciable in section-cutting ; the thread 

 of the screw regarded as formed by the hypothenuse of a right-angled 

 triangle wound upon a cylinder is merely a particular application 

 of the inclined plane of Eivet's microtome, and has the advantage of 

 being more accurate and of insuring a longer surface of contact 

 between the fixed and moving parts than is the case with the slider 

 of the latter. 



The author tested the accuracy of a microtome-screw (and that 

 one which worked loosely in its bearings), observing under a Micro- 

 scope the motion of a fine needle-point carried by the screw, using 

 an eye-piece micrometer. The displacements produced by a single 

 turn of the screw were measured for 25 turns; of these, 7 gave a 

 motion of 543 fx, 8 of 534 yu,, and 10 of 537 /x ; similarly the dis- 

 placements corresponding to each two divisions on the head of the 

 screw, which was divided into 50 parts, were in 18 cases 20*8 fx, in 

 4 cases 19*5 /a, and in 3 cases 22*2 /a. The difference of 1*3 /a 

 may reasonably be ascribed to errors of observation, and the author 

 concludes that the accuracy of the screw is all that can possibly be 

 required. Backlash may be got rid of by the use of a spring. 



Rapid Section-cutting.* — For the benefit of those who have so 

 little time for microscopic work that every minute is precious, 

 Mr. J. E. Whitney describes a contrivance for section-cutting which 

 is nearly as rapid as free-hand cutting, and yet enables really good 

 sections to be made with more certainty. Where one wishes to make 

 sections of numerous vegetable tissues for comparative study, and has 

 only a short time for the purpose, the tedious process of imbedding 

 necessary with ordinary machines is a serious obstacle. 



To avoid the necessity of imbedding the object, the author simply 

 cuts in a block of hard wood (say 3 in. by 4 in., and 1^ in. thick) a 

 wedge-shaped opening, 1;^ in. by 2 in. or thereabouts (fig. 110), into 



Fig. 110. 



which the object to be cut is placed so that its sides touch the tapering 

 sides of the opening, and prevent motion. On the top of the block 

 over which the blade of the razor is to pass cement two pieces of glass 



* Proc. Amer. Soc. Micr., 8th Ann. Meeting, 1886, pp. 122-3 (1 fig.). 



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