THE EYEPIECE MICROMETER 65 



Zeiss prepares a compensating eyepiece micrometer 

 for use with his apochromatic objectives, the divisions 

 of which are so computed that (with a tube length of 

 1 60 mm.) the value of each is equivalent to as many 

 micra as there are millimetres in the focal length of the 

 objective employed. 



Wright's Eikonometer is really a modification of 

 the eyepiece micrometer for rapidly measuring micro- 

 scopical objects by direct inspection, having previously 

 determined the magnifying power of the particular 

 optical combination employed. It is a small piece of 

 apparatus resembling an eyepiece, with a sliding eye 

 lens, which can be accurately focussed on a micrometer 

 scale fixed within the instrument. When placed over 

 the microscope ocular the divisions of this scale measure 

 the actual size of the virtual image in millimetres. 



In order to use this instrument for direct measure- 

 ment, it is first necessary to determine the magnifying 

 power of each combination of ocular, tube length and 

 objective. 



Place a stage micrometer divided into hundredths 

 of a millimetre on the microscope stage and focus 

 accurately. 



Rest the eikonometer on the eyepiece. Observa- 

 tion through the eikonometer shows its micrometer 

 scale superposed on the image of the stage micrometer. 



Rotate the eikonometer until the lines on the .two 

 scales are parallel, and make the various adjustments 

 to ensure that two lines on the eikonometer scale 

 coincide with two lines on the stage micrometer. 



For the sake of illustration it may be assumed that 

 five of the divisions on the stage micrometer accu- 

 rately fill one of the divisions of the eikonometer scale ; 

 this indicates a magnifying power of 500 as the con- 

 stant for that particular optical combination, and a 

 record should be made of the fact. 



The magnification constants of the various other 

 5 



