APPENDIX 131 



Make the lines of the two micrometers parallel by rotating the 

 ocular or changing the position of the stage micrometer, or both if 

 necessary, and then make any two lines of the stage micrometer 

 coincide with any two on the ocular micrometer. To do this it may 

 be necessary to pull out the draw tube a greater or less distance. 

 See how many spaces are included on each of the micrometers. 



Divide the value of the included space or spaces on the stage 

 micrometer by the number of divisions on the ocular micrometer 

 required to include them, and the quotient so obtained will give the 

 valuation of the ocular micrometer in fractions of the unit of meas- 

 ure of the stage micrometer. For example, suppose the millimetre 

 is taken as the unit for the stage micrometer, and this unit is divided 

 into spaces of ^ and Ytis """• If now, with a given optical 

 combination and tube length, it requires 10 spaces on the ocular 

 micrometer to include the real image of ^ mm. on the stage 

 micrometer, obviously i space on the ocular micrometer would 

 include only one-tenth as much, or ^ mm. io=yJj mm., that is, 

 each space on the ocular micrometer would include j^ of a milli- 

 metre on the stage micrometer, or -^ mm. of length of any 

 object under the microscope, the conditions remaining the same. 

 Or, in other words, it would require 100 spaces on the ocular 

 micrometer to include i mm. on the stage micrometer, then as 

 before 1 space of the ocular micrometer would have a valuation of 

 j^ mm. for the purposes of micrometry; and the size of any 

 minute object may be determined by multiplying this valuation of 

 I space by the number of spaces required to include it. For 

 example, suppose the fly's wing or some part of it covers 8 spaces 

 on the ocular micrometer, it would be known that the real size of 

 the part measured is j^ mm. x 8=Tij mm. or So/n (lS7)- 



Varying the Ocular Micrometer Valuation. — Any change in the 

 objective, the ocular, or the tube length of the microscope, that is to 

 say, any Change in the size of the real image, produces a corre- 

 sponding change in the ocular micrometer valuation (152, i6i)." 



