MICROSCOPE ACCESSORIES 217 



Filar Micrometers. — This type of micrometer con- 

 sists of an ocular fitted with a movable scale or hair 

 line, which, when a graduated drum is revolved, 

 traverses a fixed scale. The micrometer ocular 

 replaces the ocular of the microscope, and the mount- 

 ing is usually provided with a set screw to prevent 

 the apparatus turning in the draw tube. The scale 

 and drum rulings of a filar micrometer must be stand- 

 ardized against a stage micrometer in the manner 

 described in the preceding section. Filar micrometers 

 are much more accurate than the disk types; but as 

 they are integral parts of the ocular, one is limited, 

 in using them, to the single ocular in which the scale 

 is mounted. Disk micrometers may be transferred 

 from one ocular to another of different power, and one 

 scale may be standardized for use with a wide range 

 of oculars. 



The unit' of measurement in microscopy is the 

 micron (n), which is equivalent to 0.001 millimeter 

 (1/2500 inch), and the dimensions of microscopic 

 objects are expressed in microns. 



CAMERA LUCIDA 



This accessory is used in the preparation of sketches 

 or drawings of microscopic objects. The essential 

 parts of the apparatus are a prism and a mirror. 

 The reflecting apparatus is so placed that the observer 

 may see the specimen and the drawing surface at the 

 same time. In brief, the instrument serves to combine 

 the image produced by the microscope with a view of 

 the drawing surface. One of the reflecting elements 

 is a cube consisting of two triangular glass prisms 

 separated from each other by a silvered surface. 



