chap, xxviii.i MAGNIFYING POWER. 367 



i 



-|- = magnifying power. 



Tcfo 



= 50 diameters. 



If the distances marked on the micrometer be in 

 millimetres, then a millimetre scale must be used to 

 measure the distance between the two points. 



A second method consists in fitting to the eye- 

 piece of the microscope a neutral tint reflector (page 

 373). The microscope is bent so as to be placed hori- 

 zontally : on the table straight under the reflector and 

 at the nearest distance for distinct vision (10 inches) 

 is placed an inch scale or a millimetre scale, according 

 as the micrometer is ruled to give to British or French 

 measurement. The reflector is placed at an angle of 

 45 to the line of the microscope tube, and the 

 observer's eye is placed immediately above the reflector 

 and looking straight down upon it. The rays from the 

 micrometer scale, after passing out by the eye-piece, 

 fall on the reflector and are partly reflected upwards 

 into the observer's eye, who accordingly sees an image 

 of the micrometer lines. At the same time rays from 

 the scale on the table pass upwards, pass through the 

 tinted glass unaffected, and reach the eye. The image 

 of the micrometer scale and the rays from the scale on 

 the table thus coincide, and the observer can read off 

 how many divisions of the scale on the table are in- 

 cluded between two lines of the micrometer scale. He 

 thus obtains the apparent size, and can make the 

 calculation as before. 



The inch or millimetre scale might also be held at 

 the side of the microscope stage, as the compasses 

 were held, and a direct reading taken in this way of 

 the apparent size of the object. 



It need scarcely be observed that the magnifying 

 power determined by any such method is true only 

 for the particular objective and eye-piece that are in 



