ZOOLOGY AND BOTANY, MICROSCOPY, ETC. / 79 



work, as, for instance, with soft celloidin, fresh, or alcohol-hardened pre- 

 parations. The second knife " B " is the most generally useful knife. 

 It is not ground to such a fine angle, and is used with the majority of 

 paraffin preparations and also with hard celloidin. 



The microtome will cut sections measuring up to 150 x 120 mm. 

 (6" x 4|") in either paraffin or celloidin. The thickness of the sections 

 can be varied from to O-06 mm., each division on the scale being equal 

 to 0*002 mm. The total distance through which the microtome will 

 automatically feed the object-holder is 21 mm. 



PI. XV. fig. 2 shows the instrument with the knife in position for 

 cutting celloidin section. In fig. 3 the knife is seen in position for 

 cutting paraffin. 



An Eighteenth Century Microtome.* — Description of an instru- 

 ment for cutting transverse slices of wood for microscopical objects. 



A A, in No. 1 (fig. 115 f), represents a cylinder of ivory, 3i-in. long 

 and 2-in. in diameter, to the one end of which is fitted B B, a plate of 

 bell-metal, the section of which with the manner of fitting it to the 

 ivory, may be seen in 2, in which the several parts are marked with the 

 same letters as in 1. 



C is a plate of brass, fitted to the other end of the cylinder, through 

 which and the ivory there pass two long screws, which take into the 

 thick part of the bell-metal B B, so as to fix both plates strongly to the 

 ivory, into which they are also indented, so as to prevent such shaking 

 as might otherwise happen after swelling or shrinking. 



D D. The cutter, whose edge is a spiral, and the difference of whose 

 longest and shortest radii is equal to the thickness of the largest piece 

 of wood that the instrument would take in. The lowest side of this 

 cutter must be ground extremely flat and true, in order that all the parts 

 of its edge may be exactly in the same plane, and that the middle part 

 of it may be applied closely to the flat circular plane left at the centre of 

 the plate BB, to preserve it in the proper direction when carried round 

 by the handle. 



All that part of the bell-metal which the edge of the cutter traverses 

 is turned so low as not to touch it (see the section), the middle of the 

 cutter is about ^-in. thick and has in it a square hole that fits on the end 

 of a steel axis P P, one end of which turns on a pivot in the plate C, 

 the other end in the plate B B. This end has a conical shoulder which 

 fits into a hole of the same shape in the under side of the plate, as repre- 

 sented in the section. 



ee. A piece of brass somewhat in the form of an index, which is 

 also put on the axis P P ; this piece has a round hole in its centre, so 

 large as to admit of its being turned into any position with regard to 

 the cutter ; and in order to keep it concentric thereto there is left on it 

 a circular projection, which fits into a cavity made in the lower side of 

 the handle where it fits on the axis (see the section). 



* The Construction of Timber from its early Growth ; explained by the 

 Microscope and proved from Experiments, in a great variety of kinds. By John 

 Hill, M.D., Member of the Imperial Academy. London, 1774, 2nd ed., 64 pp. 

 folio (44 pis.). 



t The block for this illustration was kindly presented by Mr. C. Lees Curties. 



8 F 2 



