106 MAGNIFICATION AND MICROMETRY. \_CH. IV. 



\ 167. Remarks on Micrometry. — In using adjustable objectives (§ 22, 96), the 

 magnification of the objective varies with the position of the adjusting collar, be- 

 ing greater when the aijustment is closed as for thick cover glasses than when 

 open, as for thin ones. This variation in the magnification of the objective pro- 

 duces a corresponding change in the magnification of the entire microscope and 

 the ocular micrometer valuation— therefore it is necessary to determine the mag- 

 nification and ocular micrometer valuation for each position of the adjusting collar. 



While the principles of micrometry are simple, it is very difficult to get the ex- 

 act size of microscopic objects. This is due to the lack of perfection and uni- 



(B) By finding the number of divisions on the ocular micrometer required to in- 

 clude the image of an entire millimeter of the stage micrometer, and using this 

 number as a divisor. This number is also sometimes called the ocular micrometer 

 ratio. Taking the same case as in (A), suppose five divisions of the ocular mi- 

 crometer are required to include the image of T 2 B ths mm., on the stage micrometer, 

 then evidently it would require 5 -j- j 5 = 25 divisions on the ocular micrometer to 

 include a whole millimeter on the stage micrometer, then the number of divisions 

 of the ocular micrometer required to measure an object divided by 25 would give 

 the actual size of the object in millimeters or in a fraction of a millimeter. Thus, 

 suppose it required 15 divisions of the ovular micrometer to include the image of 

 some part of the fly's wing, the actual size of the part included would be 15 ■— 25 

 = |- or 0.6 mm. This method is really exactly like the one in (A), for dividing by 

 25 is the same as multiplying by ^V' n - 



(C) By having the ocular micrometer ruled in millimeters and divisions of a mil- 

 limeter, and then getting the size of the real image in millimeters. In employing 

 this method a stage micrometer is used as object and the size of the image of one 

 or more divisious is measured by the ocular micrometer, thus : Suppose the stage 

 micrometer is ruled in T Vth and xlrsth mm. and the ocular micrometer is ruled in 

 millimeters and T l ff th mm. Taking T 2 -th mm. on the stage micrometer as object, 

 as in the other cases, suppose it requires 10 of the jijth mm. spaces or 1 mm. to 

 measure the real image, then the real image must be magnified |§ -f- fa = 5 diame- 

 ters, that is, the real image is five times as great in length as the object, and the 

 size of an object may be determined by putting it under the microscope and getting 

 the size of the real image in millimeters with the ocular micrometer and dividing 

 it by the magnification of the real image, which in this case is 5 diameters. 



Use the fly's wing as object, as in the other cases, and measure the image of the 

 same part. Suppose that it required 30 of the 3*5 mm. divisions = f $ mm or 3 mm. 

 to include the image of the part measured, then evidently the actual size of the 

 part measured would be 3 mm. -=- 5 = f mm., the same result as in the other cases. 



In comparing these methods it will be seen that in the first two (A and B) the 

 ocular micrometer may be simply ruled with equidistant lines without regard to 

 the absolute size in millimeters or inches of the spaces. In the last method the 

 ocular micrometer must have its spaces some known division of a millimeter or 

 inch. In the first two methods only one standard of measure is required, viz., the 

 stage micrometer ; in the last method two standards must be used, — a stage mi- 

 crometer and an ocular micrometer. Of course, the ocular micrometer in the first 

 two cases must have the lines equidistant as well as in the last case, but ruling lines 

 equidistant and an exact division of a millimeter or an inch are two quite different 

 matters. 



