28 A MANUAL OF BACTERIOLOGY 



precautions to prevent external microbes gaining 

 an entrance. 



Microtomes. These instruments are used for cut- 

 ting sections of organs, tissues, etc. ; and there are 

 many forms in use. Fig. 8 represents Schanze's 

 microtome, and it is a most useful instrument for 

 cutting sections imbedded in celloidin. 



The Cambridge rocking microtome 1 is an instru- 

 ment for producing ribbons of sections imbedded 

 in paraffin. The razor is supported and clamped in 

 front of a brass tube containing the imbedded 

 object. This tube fits tightly on to the end of a 

 cast-iron lever ; and is made to slide backwards or 

 forwards so as to bring the imbedded object near 

 to the razor. By an arrangement of pivots, milled 

 screws, and a milled wheel, the lever is moved for- 

 wards, and the object to be cut is therefore brought 

 across the edge of the razor : when the lever is made 

 to move backwards the section is cut. The values 

 of the teeth on the milled wheel are as follows : 



1 tooth of the milled wheel = ^57 of an inch = '000625 mm. 



2 teeth ,, ,, = ^J^ ,, ='001250 mm. 

 4 j, ,, ,, = 10000 55 '0025 mm. 



16 ,, ,, 15= urfto 55 = '01 mm. 



On working this microtome the sections should 

 adhere together so as to form a ribbon. The work- 

 ing of this instrument requires very little skill on 

 the part of the operator ; consequently it is to be 

 recommended to those who require very thin sec- 



1 Made by the Cambridge Scientific Instrument Company, 

 St. Tibb's Row, Cambridge. 



