28 MICROSCOPE AND ACCESSORIES [CH. I 



50. 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 appears within the 

 light circle, and by moving the object, if it is of sufficient size, differ- 

 ent parts are brought successively into the field of view. 



In general, the greater the magnification of the entire microscope, 

 whether the magnification is produced mainly by the objective, the 

 ocular, or by increasing the tube length, or by a combination 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 diaphragm. 

 Some oculars, as the orthoscopic and periscopic, are so constructed as 

 to eliminate the ocular diaphragm, and in consequence, 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. 



4 5 



17 



85 mm 



FIG. 37. Figures showing approximately the actual size of the field with ob- 

 jectives 0/85 mm., 45 mm., 77 mm., 3 mm., and 2 mm., equivalent focus, and 

 ocular of 3jY 2 mm., equivalent focus in each case. This figure shows graphically 

 what is also very clearly indicated in the table ( \ 52}. 



51. The size of the field of the microscope as projected into the 

 field of vision of the normal human eye (i. e., the virtual image) may 

 be determined by the use of the camera lucida with the drawing surface 

 placed at the standard distance of 250 millimeters (Ch. IV.) 



52. Table showing the actual size in millimeters of the field of a 

 group of commonly used objectives and oculars. Compare with the graphic 

 representation in Fig. 37. See also 50. 



