3° 



FIELD OF A MICROSCOPE 



[Ch. 1 



object under the microscope, the field appears as a disc of light. When 

 examining an object it appears within the Ught circle, and by moving 

 the object, if it is of sufficient size, different parts are brought succes- 

 sively 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 (§ 235), 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 eUminate the ocular diaphragm, and in consequence, although 

 this is not the sole cause, the field is considerably increased. 



§ 48, Measuring the size of the field. — Use a stage micrometer 

 (fig. 80) as object, and read off the number of spaces required to meas- 

 ure the diameter of the light disc as seen in the microscope. Use 

 first a low objective (16 mm.) and a low ocular (4X or Sx), then use the 

 higher ocular (8x or lox). Do the same with the 4 or 8 mm. objective 

 and the two oculars. Make a table giving the diameter of the field 

 in each case and compare with the accompanying table. The tube- 

 length (fig. 25) should be 160 mm. when making the measurements. 

 To see the effect of lengthening the tube, pull it out till the tube-length 

 is 200 mm. and note the effect on the size of the field with one objective 

 and the two oculars. (The longer the tube the smaller the field). 



§ 49. Table showing the actual size of the field of view of various objec- 

 tives and oculars with a tube-length of 160 mm. 



