INVERTEBRATA, CRYPTOQAMIA, MICROSCOPY, ETC. 



1035 



tome, for amateur use, made by Mr. A. A. Pearson, of Leeds. The 

 instrument is shown in Figs. 108 and 109, and was thus described by 

 Mr. Teosdale : — 



"In general form it is based upon the Continental model of Dr. 

 Schieflferdecker, of Strassburg,* but it is more compact, and small 

 objects are more readily held or packed, as the holding or grasping 



Fig. 108. 



Fig. 109. 



is eflfected by jaws closing centrally like those of an American 

 chuck. The object so held is raised by a double-threaded screw 

 of 50 per in., metrically equal to (but possessing some advantage 

 over) one of 25 threads per in. Its elevation is regulated and recorded 

 by a circular plate and spring stop catching in 20 equal divisions, 

 each one of which indicates an advance of -^—^ in. In practice I have 

 generally found about ^lx)> ^^ ^^'^ divisions of this graduation, the 

 most convenient average thickness for vegetable sections. 



I have used most of the forms of ordinary section-cutters for nearly 

 twenty years. Some are ingeniously contrived and good in theory, 

 or suitable for some special purpose, but have some practical fault 

 or inconvenience for the general all-round work of an amateur. 

 The Strassburg model which furnished the basis of the design of 

 this, was, it appeared to me, an excellent one, and the principle of its 

 construction sound, inasmuch as the object was not irregularly forced 

 up by pressure from beneath, but held firm, and liad the cutting table 

 or guide-plate lowered by screw and graduated index. Such intelli- 

 gent mechanical aid to accuracy and facility ought, and would, 

 assuredly have obtained favourable recognition and general adoption, 

 but, uufortiniately, in copies of it made and distributed in England 

 the screw was cut taper in the lathe (instead of being cut perfectly 

 parallel with stocks and ilies), consequently the instrument was some- 

 wliat shaky." 



* ' Quart. Joiirn. Mior. Sci.' (1877) p. n.'i. 



