44 MAGNIFICA TION AND DRA WING. 



croscope (§§ 32, 37) for the stage micrometer. The image of the stage 

 micrometer will appear to be directly under or upon the ocular microme- 

 ter. 



Make the lines of the two micrometers parallel by rotating the dbular, 

 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 mi- 

 crometer 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 measure of the stage 

 micrometer. For example, suppose the millimeter is taken as the unit 

 for the stage micrometer and this unit is divided into spaces of y^th and 

 Y^th millimeter. If now, with a given optical combination and tube- 

 length, it requires 10 spaces on the ocular micrometer to include the 

 real image of yVth millimeter on the stage micrometer, obviously one 

 space on the ocular micrometer would include only one-tenth as much, or 

 xVth mm. -7- 10 = y-jjiyth mm. That is, each space on the ocular microme- 

 ter would include yihyth of a millimeter on the stage micrometer, or nnj-th 

 millimeter 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 millimeter on the stage micrometer, 

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

 of yxi^th millimeter for the purposes of micrometry ; and the size of any 

 minute object may be determined by multiplying this valuation of one 

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

 suppose the fly's wing or some part of it covered 8 spaces on the ocular 

 micrometer, it would be known that the real size of the part measured 

 is T foth mm. x 8 = T f^th or 80^ (§ 108). 



§ 115. 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 (§ 103, 112). 



§ 116. Micrometry with the Ocular Micrometer. — Use the 3 mm. 

 objective and preparation of Necturus blood corpuscles as object. 

 Make certain that the tube of the microscope is of the same length as 

 when determining the ocular micrometer valuation. In a word be sure 

 that all the conditions are exactly as when the valuation was deter- 

 mined, then put the preparation under the microscope and find the 

 same three red corpuscles that were measured in the other ways (§§ 1 10, 

 in). 



