152 THE USE OF THE MICROSCOPE 



camera. Another accurate method is to photograph 

 both on the same plate. In all such measurements the 

 mechanical tube length should be 160 (or 170) millimeters, 

 if possible. In making a series of measurements with an 

 eyepiece micrometer, the tube length is often previously 

 altered to make a certain number of divisions of the eye- 

 piece scale coincide with a definite number of divisions 

 of the object scale. If these numbers are properly chosen, 

 little shifting of the tube length is necessary. Even this 

 can probably be obviated if the actual size of the divisions 

 of the eyepiece scale is known, as well as the diameter of the 

 eyepiece diaphragm. To get the magnification of an 

 objective, use an eyepiece scale of known intervals, in an 

 eyepiece which has the diaphragm below the lenses, along 

 with a stage micrometer. The magnification of the 

 objective is equal to any extent of the eyepiece scale divided 

 by the coinciding portion of the object scale. 



Summary. — A high eyepoint is important, especially 

 for wearers of spectacles. Eyepieces with small eyelenses 

 and close eyepoints are especially to be avoided by those 

 who need spectacle glasses. The 10- times compensating 

 eyepiece of Zeiss, for example, has a comfortably high 

 eyepoint. The higher compensating and orthoscopic eye- 

 pieces with external diaphragms have large eyelenses and 

 high eyepoints. Compensating eyepieces can be used 

 with all achromatic objectives (with the lower as well 

 as with the higher powers) if a deep-green screen, such as 

 Wratten No. 58, is also employed. All eyepieces by the 

 same maker should be parfocal, or the tube length is not 

 kept constant. Eyepieces by different makers may not 

 agree in position of the focal plane and, hence, should be 

 perhaps restricted to their own objectives. 



Practical Points 



1. Have a special corrected eyepiece (orthoscopic) for ocular 

 magnifications much over 10, when achromatic objectives of low 

 and medium power are much used. 



