134 



HOW TO SEE WITH THE MICROSCOPE. 



to be forty-eight linear, and substituting this value, we 

 have 



Focal length 



48+12-50 = 600 

 2401 



= ^- = "25 in. again. 



Either computation gives the same results (rejecting 

 fractions too small to affect things sensibly), and we 

 see, too, that the glass sold for the one-fifth is really 

 but a true fourth; and, furthermore, it is evident that 

 a change in the distance of the two micrometers in 

 other words, working with different lengths of tubes 

 does not practically affect the results. 



TABLE OF THE MAGNIFYING TOWERS OF SINGLE CONV6X 



LENSES. 



The first column gives the focal length for parallel 

 rays; the second, third, fourth and fifth, give the mag- 

 nifying powers at ten, twelve and a half, twenty-five, 

 and fifty inches respectively, the last three being from 

 the American Journal of Science and Arts, as before 

 mentioned, while the second was computed by the Hon. 

 J. D. Cox, of Ohio, and by him presented to the 

 author. 



