ILLUMINANTS 67 



As in the previous method, the value of each division 

 of the micrometer scale (i. e., the comb) must first be 

 determined for each optical combination. This is 

 effected as follows: 



1. Place the filar micrometer and the stage microm- 

 eter in their respective positions. 



2. Rotate the screw of the filar micrometer until the 

 movable wire coincides with the fixed one, and the 

 index marks zero on the drum head. (If when the 

 drum head is at zero the two wires do not exactly coin- 

 cide they must be adjusted by loosening the drum screw 

 and resetting the drum.) 



3. Focus the scale of each micrometer accurately, 

 and make the lines on them parallel. 



4. Rotate the head of the micrometer screw until 

 the movable line has transversed one division of the 

 stage micrometer. Note the number of complete revo- 

 lutions (by means of the recording comb) and the frac- 

 tions of a revolution (by means of scale on the head 

 of the micrometer screw) , which are required to meas- 

 ure the o.o i mm. 



5. Make several such estimations and average the 

 results. 



6. Note the optical combination employed in this 

 experiment and record it carefully, together with the 

 micrometer value in terms of /*. 



7. Repeat this process for each of the different 

 optical combinations and record the results. 



To measure an object by this method, simply note 

 the number of revolutions and fractions of a revolu- 

 tion of the screw-head required to traverse such object 

 from edge to edge, and express the result as micro, 

 by reference to the recorded values for that particular 

 optical combination. 



Microscope Illuminant. In tropical and subtropical 

 regions diffuse daylight is the best illuminant. In 

 temperate climes however daylight of the desirable 



