46 MAGNIFICA TION AND DRA WING. 



§ 117. Remarks on Micrometry. — In using adjustable objectives {\\ 16, 63), the 

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

 ing greater when the adjustment 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- 

 formity of micrometers, and the difficulty in determining the exact limits of the 

 object to be measured. Hence, all microscopic measurements are only approxi- 

 mately correct, the error lessening with the increasing perfection of the apparatus 

 and the skill of the observer. 



A difficulty when one is using high powers is the width of the lines of 

 the micrometer. If the micrometer is perfectly accurate half the width of 

 each line belongs to the contiguous spaces, hence one should measure the image 

 of the space from the centers of the lines bordering the space, or as this is some- 

 what difficult in using the ocular micrometer, one may measure from the inside of 

 one bordering line and from the outside of the other. If the lines are of equal 

 width this is as accurate as measuring from the center of the lines. Evidently it 

 would not be right to measure from either the inside or the outside of both lines 

 (PI. Ill, Fig. 29). 



It is also necessary in micrometry, to use an objective of sufficient power to en- 

 able one to see all the details of an object with great distinctness. The necessity 

 of using sufficient amplification in micrometry has been especially remarked upon 

 by Richardson, Monthly Micr. Jour., 1874, 1875 ; Rogers, Proc. Anier. Soc. Micro- 

 scopists, 1SS2, p. 239; Ewell, North American Pract., 1890, pp. 97, 173. 



As to the limit of accuracy in micrometry, one who has justly earned the right 

 to speak with authority expresses himself as follows : " f assume that o 2^1 is the 

 limit of precision in microscopic measures, beyond which it is impossible to go 

 with certainty.'" W. A. Rogers, Proc. Amer. Soc. Micrs., 1883, p. 198. 



scope and getting the size of the real image in millimeters with the ocular mi- 

 crometer 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 T V 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. -j- 5 = § 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 oc- 

 ular 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 ocu- 

 lar 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 methods two standards must be used, — a stage micrometer 

 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 equi- 

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

 matters. 



