APPENDIX 147 
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 7; and 735 mm. 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 1 space on the ocular micrometer would 
include only one-tenth as much, or 7, mm. 10o=;4, mm., that is, 
each space on the ocular micrometer would include ;4, 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 1 mm. on the stage micrometer, then as 
before 1 space of the ocular micrometer would have a valuation of 
tis 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 y35 mm. X 8=7$5 mm. or 80,4 (157). 
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, 161).” 
