Ii6 MAGNIFICATION AND MICROMETRY \_CH. IV 



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 in 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-Q-g-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 yVth mm. -=-10 = y-J^th mm. That is, each space on the ocular mi- 

 crometer would include y-^oth of a millimeter on the stage micrometer, 

 or y-Jijth millimeter of the length of any object under the microscope, 

 the conditions remaining the same. Or, in other words, it would re- 

 quire 100 spaces on the ocular micrometer to include i millimeter on the 

 stage micrometer, then as before, i space of the ocular micrometer would 

 have a valuation of y^-Q-th millimeter for the purposes of micrometry. 

 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 y^th mm. x 8 = y-fo- mm. or 80 ju. ( 166). See 

 Mark, Jour. Applied Microscopy, Vol. I, p. 4. 



173. Micrometry with the Ocular Micrometer. Use the 3 

 mm. (-|- in. ) objective with the 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 determined, then put the preparation under the microscope and 

 find the same three red corpuscles that were measured in the other 

 ways (168-169). 



Count the divisions on the ocular micrometer required to enclose 

 or measure the long and the short axis of each of the three corpuscles, 

 multiply the number of spaces in both cases by the valuation of the 

 ocular micrometer for this objective, tube-length and ocular, and the 

 results will represent the actual length of the axes of the corpuscles 

 in each case. 



