142 A MANUAL OF BACTERIOLOGY 



glass, oil, etc., and they may not form a truly homogeneous 

 whole, for the finest work the correction collar is still 

 desirable. So much for the high-power objectives. As 

 regards the lower powers, which, of course, are dry, a 

 f-in. and a |-in. are generally selected. The f-in. is a 

 more serviceable lens than the 1-in. which is often recom- 

 mended. A very useful accessory is a " double " or 

 " triple nosepiece." This consists of a light metal frame- 

 work, which is attached to the lower end of the tube of 

 the microscope, on to which two or three objectives can 

 be screwed. The framework can be rotated, thus bringing 

 each objective in succession into the optical axis of the 

 instrument, and the necessity for unscrewing and screwing 

 on each time an objective is changed is obviated. A 

 microscope such as described, with sub-stage condenser, 

 two eyepieces, a f-in. and a J-in. dry and a ^--in. oil- 

 immersion objectives, triple nosepiece, etc., complete in 

 case, can be obtained for about 15, and it is well to add 

 another sovereign or two for superior finish. Both British 

 and Continental firms supply microscopes arranged as 

 indicated, and in this department the English makers 

 hold their own. 



The measurement of micro-organisms is carried out by 

 means of a stage micrometer, alone, or in combination 

 with an eyepiece micrometer. The former consists of 

 a scale of tenths and hundredths of a millimetre or 

 hundredths and thousandths of an inch ruled in fine lines 

 on a glass plate, by means of which the measurements 

 can be made by focussing the scale under the microscope. 

 The stage micrometer is placed in position on the stage 

 and the scale is focussed with the particular ocular, 

 objective, and tube length which are to be used. A 

 drawing of the scale is made with a camera lucida ; the 

 micrometer is then removed and the object placed in 

 position and a second drawing is made of the object on the 



