464 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS 



edge clear of the object. (2) It should in general be less tilted for 

 hard and brittle objects than for soft ones, therefore, cmteris paribus, 

 less for paraffin than for celloidin. (3) The extent of useful tilt 

 varies (according to the angle to which the knife is ground, amongst 

 other factors) between and 16. (Jung's ordinary knife-holders 

 have mostly a tilt of about 9, which is only enough, with the usual 

 plane-concave knives, for cutting ribbons of sections with hard 

 paraffin.) (4) Excessive tilt causes paraffin sections to roll, and may 

 produce longitudinal rifts in them. It may also set up vibrations in 

 the blade, which are heard as a humming tone, and which give an 

 undulatory surface to the sections. Excessive tilt may often be 

 recognised by the knife giving out a short metallic note just as it 

 leaves the object. For knives with plane under-surfaces it is seldom 

 advisable to give less than 10 tilt; whilst knives with concave 

 under-surfaces on the contrary may require to be placed almost 

 horizontal. A knife with too little tilt will cut a second section, 

 or a portion of one, without the object having been raised ; showing 



FIG. 393. Object-holder with jaws. 



that during the first cut the object was pressed down by the knife 

 and recovered itself afterwards. This fault is denoted by the ringing 

 tone given out by the knife on passing back over the object before it 

 is raised. Ribbon-cutting requires a relatively hard ' paraffin and 

 less tilt. With celloidin it is very important to avoid insufficient 

 tilt, as the elastic celloidin, with too little tilt, yields before the 

 knife and is not cut. 



The exigencies of section-cutting have given rise to a great rar !>'/</ 

 of object-holders in this instrument. The simplest is seen in O S, 

 fig. 391, which is a pair of jaws clamped by screws and fixed upon 

 the pivot St by the milled head a. At n is the vernier, which indi- 

 cates the position on the mm. scale, Th, and t is an agate highly 

 polished, upon which the micrometer screw m works to drive forward 

 the object-carrier, O S. 



The Zoological Station at Naples employs a holder specially de- 

 signed for use with paraffin ; the object is soldered with paraffin on 

 to the cylinder, b y, fig. 392. This is supported on gimbals and may 



