The Magnifying Power of the Microscope. By A. E. Wright. 287 



lines which furnishes to us a median intercostal line. We can 

 then by the application of a scale measure the linear distance 

 between this pair of lines. 



We can now place before us another series of paired lines, 

 lying at distances apart smaller than the paired lines previously 

 considered in the ratio which will, we estimate, correspond to the 

 magnification achieved by the use of lens. We can again select from 

 among this series of paired lines that pair of lines which furnishes 

 as viewed through the diffraction grating, held at the same angle 

 as before, a median intercostal line. 



Inasmuch as the pair of lines which is seen by the unaided 

 eye from a distance of 10 in., and the pair of lines which is seen 

 through the magnifying system yield in each case retinal images of 

 precisely the same dimensions, it will be obvious that the magni- 

 fying power of the optical system will be arrived at by dividing 

 the interval between the object lines, which have been viewed 

 through the magnifying system, into the interval between the lines 

 which have been viewed by the unaided eye. 



While the procedure as described above is admirably adapted 

 to the measurement of the magnifying power of pocket-lenses and 

 such like, it would, if applied without modification to the measure- 

 ment of the magnifying power of the Microscope, involve the 

 employment of a special stage micrometer with lines ruled at 

 progressively increasing intervals. 



The difficulty which has just been adverted to can be evaded. 

 As an alternative to varying the linear distance between the rulings 

 to conform to the elongation of the flanking line, we can accom- 

 modate the elongation of the flanking lines to the periodical 

 interval of the rulings of the stage micrometer, and to the 

 magnifying power of the optical system through which we view 

 those rulings. 



The required accommodation of the elongation of the flanking 

 lines can be effected by employing, as the case may be, a more finely 

 or more coarsely ruled diffraction grating, or, more simply, by rota- 

 ting the grating in such a way as to bring — according as we desire to 

 increase or diminish the elongation — the long axis of the slit aper- 

 tures of the grating into parallelism with the rulings, or, as the case 

 may be, with the normal to the rulings. Having regulated in this 

 way the elongation of the flanking lines until we have achieved the 

 doubling of the ruling by the intercalation of an intercostal line 

 between every two principal lines, we proceed in all essential 

 points as before. In other words — maintaining the orientation 

 of the diffraction grating, which gives a central intercostal line 

 between the ruling of the stage micrometer — we view through this 

 with the unaided eye a series of paired lines placed at a distance of 

 10 in., and we arrive at the magnifying power of our optical system 

 by dividing the linear distance between the rulings on the stage 



x 2 



