NOMENCLATURE OF OBJECTIVES 133 



six-hundredths, and having brought the lines of the 

 stage micrometer in focus, we then measured the dis- 

 tance from micrometer to micrometer, finding the same 

 to be just ten inches, and on comparing the divisions of 

 the stage micrometer with those of the eye -piece, we 

 found those of the stage micrometer to be amplified 

 eighteen linear. Now, working out the formula, and 

 substituting for the symbols their proper values, we 

 have 



Focal length= !?12 = 1? =-5 in. or i inch. 



Hence, we find the glass exactly to the rate given it by 

 its maker. 



(2.) In a similar manner it was desirable to test a 

 (so-called) one-fifth objective. Proceeding just as be- 

 fore, simply changing objectives, we measured again the 

 distance from micrometer to micrometer, getting again 

 just ten inches a matter of chance, however, and one 

 not likely often to occur. Comparing the two microm- 

 eters again, we get an amplification of the stage lines 

 of thirty-eight linear. Substituting values as before 

 we now have 



Focal length = - 3 - 8 I5 = -M. = .25 in. or i inch. 



oo^ lo^il 



For the purpose of developing the capacity and value 

 of the formula, we will now repeat this example, but 

 in place of using a distance of ten inches, we will 

 arrange to secure in lieu thereof a distance of twelve 

 and one-half inches. We now find the amplification, as 

 shown by the two micrometers at this greater distance, 



