for the different objectives and for the length of tube has been 

 determined accurately, it is usually preferable. 



The values of the spaces in the eye-piece micrometer are 

 ascertained by comparison with known values of the spaces on 

 a standard stage micrometer ; for example, if one space in the 

 eye-piece micrometer corresponds to five spaces of the stage 

 micrometer, and the latter has a value of one thousandth of a 

 millimeter, each space of the former equals five thousandths of a 

 millimeter. 



The unit of microscopic measurement is the " micro-milli- 

 meter," 1 one thousandth of a millimeter. It is expressed by 

 the Greek //.. 



12. Drawing. An image of the object under the microscope 

 may be cast by reflection upon paper at the side of the micro- 

 scope, by means of a Camera lucida. Several forms of the 

 Camera lucida are adapted to use with the tube of the micro- 

 scope in a vertical position, and are more convenient for the 

 majority of cases coming within the scope of the present work. 

 Oberhauser's, Milne Edwards's, and Abbe's are of this kind. 



13. Polarizing apparatus. This is of great use in the exami- 

 nation of certain contents of cells. It consists of two Nicol 

 prisms, one below the stage of the microscope and receiving 

 the light which is reflected from the mirror, the other in the 

 eye-piece. Upon turning one of the prisms, distinctive op- 

 tical characters, not otherwise seen, are presented by grains of 

 starch, etc. 



14. Media and reagents. The fluid in which a microscopic 

 specimen is submitted to examination is technically known as its 

 medium. Chemical agents subsequently added for the purpose 

 of producing changes by which the chemical character of the 

 objects may be recognized, are termed reagents. Some of the 

 media, however, in common use produce characteristic changes 

 in certain cases, and might be as truly referred to the latter 

 class as several of the reagents themselves. The substances in 



1 For convenience of reference, the following table of comparative measure- 

 ments is given : 



(u INCHES. INCHES. M- 



6 000236 



7 000276 



8 000315 



9 000354 



10 000394 



One meter = 39.370432 inches. 



2.5399 

 T^nr = 25.3997 



253.9972 



