MICROMETRY 6 1 



particular make of condenser but generally i mm. and 

 specially thin cover-glasses (not more than 0.17 mm. 

 The objective must not have a higher NA than i.o, 

 consequently immersion lenses must be fitted with an 

 internal stop to cut down the aperture. 



Micrometer. Some form of micrometer for the pur- 

 pose of measuring bacteria and other objects is also 

 essential. Details of those in general use will be 

 found in the following pages. 



Object Marker (Fig. 54). This is an exceedingly 

 useful piece of apparatus. Made in the form of an 

 objective, the lenses are replaced by a diamond point, 

 set slightly out of the centre, which 

 can be rotated by means of a milled 

 plate. Screwed on to the nosepiece 

 in place of the objective, rotation of 

 the diamond point will rule a small 

 circle on the object slide to perma- 

 nently record the position of an in- 

 teresting portion of the specimen. 

 The diamond is mounted on a spring 

 which regulates the pressure, and 



the size of the circle can be adjusted FlG - 54- Diamond Ob- 

 ject marker, 

 by means of a lateral screw. 



METHODS OF MICROMETRY. 



The unit of length as applied to the measurement 

 of microscopical objects is the one-thousandth part of 

 a millimetre (o.ooi mm.), denominated a micron (some- 

 times, and erroneously, referred to as a micro-milli- 

 metre), and indicated in writing by the Greek letter 

 IJL. Of the many methods in use for the measurement 

 of bacteria, three only will be here described, viz. : 



(a) By means of the Camera Lucida. 



(6) By means of the ocular or Eyepiece Micrometer. 



(c) By means of the Filar Micrometer (Ramsden's 

 micrometer eyepiece) . 



