ZOOLOGY AND BOTANY, MICROSCOPY^ ETC. 301 



expected from the perfecting and development of the teehnical 

 methods of preparing, hardening, soaking, and imbedding the tissues. 

 Personally, he feels sure that any tissue (excluding bone and teeth 

 before decalcification) may be prepared so as to be cut to any degree 

 of delicacy down to • 002 mm. The microtome will work with suffi- 

 cient exactness to permit this, but hitherto there are only a few tissues 

 which we can prepare so perfectly as to admit sections of such 

 extreme minuteness. The following are the points to which he most 

 especially wishes to draw attention : — 



Sliding microtomes are in general constructed for cutting sections 

 of tissues previously hardened in alcohol, picric acid, chromic salts, 

 and other agents. Fresh tissues are decidedly better cut by freezing 

 microtomes — for instance, on the simple and practical instrument of 

 Hughes and Lewis. The addition of a freezing apparatus to a 

 thoroughly exact sliding microtome is neither advisable nor necessary. 

 The differences of temperature produced in different parts of the in- 

 strument would be apt to interfere with the perfect planeness of the 

 sliding surfaces; whilst, on the other hand, section-cutting with 

 frozen tissues is so simple and easy with the ordinary freezing appa- 

 ratus that any further complication in the way of a sliding support of 

 the knife is superfluous. 



In cutting, the microtome is to be placed before the operator as in 

 fig. 51, with the sliding surfaces abundantly covered with oil (bone- 

 oil), and the knife moistened with alcohol. In many cases, it will be 

 sufficient to simply place the hardened specimen between the arms 

 of the clamp attached to the carrier b (fig. 51). The clamp should 

 then be fixed in such a position that the specimen is as near as 

 possible to the knife-carrier. The knife will generally have to be 

 adjusted so as to bring the whole length of its blade into action. 

 Very hard specimens are frequently cut with less difficulty by 

 placing the knife more obliquely in regard to the long diameter of 

 the instrument. 



The inclination of the oblique plane upon which the carrier h 

 slides is 1 : 20, and, consequently, the section will be l-20th mm. thick 

 if the carrier is moved 1 mm. on the oblique plane. A scale in milli- 

 metres with a vernier allows the operations to be exactly regulated. 

 The vernier will be found sufficient for sections of 0"015 mm. Sec- 

 tions of greater delicacy should always be made by using the micro- 

 meter-screw (c, fig. 61), which was designed to obtain the utmost 

 exactitude in the management of the carrier b. Fig. 53 shows it on a 

 larger scale. 



The carrier c' slides on the same oblique plane as the carrier 6 

 which holds the specimen. In all positions of the latter, it is there- 

 fore possible to bring the point of the micrometer-screw g g close to 

 a small polished plate of agate /, which is fixed to the carrier b. 

 In this position, c' should be firmly screwed to the stand of the 

 microtome by d, and every revolution of the micrometer-screw g g will 

 then push the carrier 6 0"3 mm. The periphery of the drum t, 

 which is firmly attached to the screw g g, is divided into 15 equal 

 parts; and consequently each division marks a thickness of section 



