CH. IV] 



MAGNIFICATION AND MICROMETRY 



133 



§ 190. Obtaining the Valuation of the Filar Micrometer. — 

 This micrometer (Figs. 1 18-120) usually consists of a Ramsden's 

 ocular and cross lines. As seen in Fig. 119 A there are three lines. 

 The horizontal and one vertical line are fixed. One vertical line 

 may be moved by the screw back and forth across the field. 



For obtaining the valuation of this ocular micrometer an ac- 



Fig. 1 1 S. Ocular Screw-Micrometer 

 with compensation ocular X 6. The upper 

 figure shows a sectional view of the ocular 

 and the screw for moving the -micrometer 

 at the right. At the left is shown a 

 clamping screw to fasten the ocular to the 

 upper part of the microscope tube. Below 

 is a face view, showing the graduation on 

 the wheel. An ocular micrometer like this 

 is in general like the cob-web micrometer 

 and may be used for measuring objects of 

 varying sizes very accurately. With the 

 ordinary ocular micrometer very small 

 objects frequently fill but a part of an inter- 

 val of the micrometer, but with this the 

 movable cross lines traverse the object [or 

 rather its real image) regardless of the mi- 

 nuteness of the object. [Zeiss' Catalog.) 



must be magnified ^-§-=-^=5 diameters, that is, the real image is five 

 times as great in length as the object, and the size of an object may be deter- 

 mined 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 mag- 

 nification 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 j\ mm. divisions = 

 f$ mm. or 3 mm. to include the image of the part measured, then evidently 

 the actual size of the part measured is 3 mm. -=-5=! mm., the same result as 

 in the other cases. See also \ 195-196 on the Eikonometer. 



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

 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 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 equidistant is quite a different matter from get- 

 ting them an exact division of a millimeter or of an inch apart. 



