THE MICROSCOPE. 139 



piece micrometer represents ^y = .00172 mm. = 1.72 //. 

 If, therefore, a bacillus is examined with this objective, and 

 occupies three divisions on the eye-piece scale, it is 1.72 x 3 

 or 5.16 ,'j. long. 



When the micrometer value of an objective is once as- 

 certained, in the manner described, it becomes a very easy 

 matter to determine the exact size of an object by means of 

 the ocular micrometer. It should be remembered, however, 

 that this micrometer must always be placed in the same 

 eye-piece with which the original measurements were made. 

 Likewise, only the particular objectives tested should be 

 employed in such measurements. The draw-tube, it is un- 

 derstood, should be drawn out when measuring an object, 

 to the same distance as when the micrometer value was 

 determined (160 mm.) 



When an ocular screw-micrometer is employed, owing 

 to its height, the draw-tube must be lowered, so that there 

 is a distance of 170 mm. between the upper surface of the 

 ocular and the shoulder of the objective. The value of a 

 division on the screw is ascertained, for each objective, by 

 means of a stage micrometer in the manner described 

 above. 



The approximate size of an object can be determined 

 without the use of a micrometer, by dividing the length of 

 the image, as projected on the table (p. 128), by the mag- 

 nification of the objective and eye-piece used. The latter 

 is obtained in the manner described on p. 128, or from the 

 table supplied by the maker of the instrument. 



Draiving an object. The reproduction of objects seen 

 under the microscope, constitutes a most important training 

 of the student. The object should be drawn, so as to have 

 the size of the image reproduced. This can only be done 

 by projecting the image on the drawing paper, and then 

 sketching in the outline. The paper is placed on the table, 

 on the right side of the microscope. On looking into the 



