32 



MICROSCOPE AND ACCESSORIES 



CM. /] 



by getting the object in the center of the field with a low objective 

 before putting the high objective in position. This is greatly facili- 

 tated by using a nose-piece, or revolver. (See Fig. 40 and the 

 pictures of microscopes, Ch. II.) 



Fig. 4t. Krauss' Method of 

 Marking Objectives on a Re- 

 volving Nose-Piece. 



As seen in the figure, the 

 equivalent focus of the objective 

 is engraved on the diaphragm 

 above the back lens and may be 

 very readily seen in rotating the 

 nose-piece. This is of great 

 advantage, as one can see what 

 objective is coming into place 

 without trouble. It is also an 

 advantage in showing where 

 each objective belongs when the 

 microscope comes from the 

 manufacturers. The method 

 is coming into general use. 



Field or Field of View of a Microscope. — This is 

 the area visible through a microscope when it is in focus. When 

 properly lighted and there is no object under the microscope, the 

 field appears as a circle of light. When examining an object it ap- 

 pears within the light circle, and by moving the object, if it is suffi- 

 cient size, different parts are brought successively into the field of 

 view. 



In general, the greater the magnification of the entire micro- 

 scope, whether the magnification is produced mainly by the object- 

 ive, the ocular, or by increasing the tube length, or by a combina- 

 tion of all three (see Ch. IV, under magnification), the smaller is 

 the field. 



The size of the field is also dependent, in part, without regard 

 to magnification, upon the size of the opening in the ocular dia- 

 phragm. Some oculars, as the orthoscopic and periscopic, are 

 so constructed as to eliminate the ocular diaphragm, and in conse- 

 quence, although this is not the sole cause, the field is considerably 

 increased. The exact size of the field may be read off directly by 

 putting a stage micrometer under the microscope and noting the 

 number of spaces required to measure the diameter of the light circle. 



§ 57- 



