1 892-93.] On Microscopical Measurement. 81 



equal to half an inch, the magnification would be 50 times, 

 and so on. 



It will be seen from the above that the question of the size 

 of an object and the magnifying power of the instrument are 

 two totally different things. Some try to find out the latter 

 in a simple way, by looking through the microscope with one 

 eye, while they glance with the other eye at a foot-rule placed 

 at the distance of ten inches. I should recommend no one to 

 do this, because in a great many cases the right and left eyes 

 are not of the same focal length, and the values obtained in this 

 way may be quite illusory. Ten inches is the recognised focal 

 length of a normal human eye, and that is why the virtual 

 image requires to be projected at that distance. 



Thirdly, there is another measurement to be noticed — viz., 

 How to ascertain the magnifying power of each object-glass by 

 itself alone — or, as it is termed, its initial magnifying power. 

 To do this, we require again a stage micrometer, and in this 

 case we also require an eyepiece micrometer, both accurately 

 ruled to a given scale — say for the stage one, a millimetre 

 ruled into a hundred divisions, and for the eyepiece one, five 

 millimetres ruled into fifty parts. Place on the microscope 

 any object-glass you wish to measure, the stage micrometer 

 being on the stage, and the eyepiece micrometer in the eye- 

 piece. Now remove the field lens of the eyepiece, and lay it 

 aside, as you require the eyepiece with the eye lens only 

 remaining in it. The image of the divisions of the stage 

 micrometer as magnified by the object-glass reach the eye- 

 piece micrometer without the intervention of the field lens, 

 and their magnified image can be got at once. Suppose five 

 divisions of the stage micrometer cover ten divisions in the 

 eyepiece — as these eyepiece divisions are ten times coarser 

 than the stage ones, multiply them by ten, and divide by the 

 number they represent on the stage divisions, and you at once 

 get the power of your object-glass by itself, thus — 



10X10 



So that the object-glass upon which this experiment was tried 

 has a magnification of twenty times by itself alone, and is 

 therefore what is termed a half - inch objective. In this 



VOL. III. F 



