173 



dent with a line of the latter, using care to see that 

 they are parallel. As the lines of the stage micro- 

 meter will appear to have a certain amount of 

 thickness, make the first line of the eyepiece 

 micrometer correspond with one edge of a line on 

 the other. Now read off how many of the lines 

 are contained in one space of the stage micrometer 

 and note this. We will assume that it is 8 divis- 

 ions. Replace the stage micrometer by the object 

 to be measured and bring one edge of the object 

 coincident with the first line of the eyepiece 

 micrometer, being careful to leave all the con- 

 ditions unchanged. Note how many divisions are 

 required to cover the object and divide by the 

 figure first obtained with the stage micrometer. 

 Thus, if there are 40 spaces which we know are 

 o.l mm. divisions the real size of the object will be 

 0.5 mm., or forty tenths divided by eight. 



If measurements are made under exactly the 

 same conditions of tube length, with same objec- 

 tives, it will be unnecessary to repeat the opera- 

 tion with eyepiece and stage micrometer, as the 

 proper ratio may be marked on a card, as it 

 remains constant. 



The most efficient apparatus however, for obtain- 

 ing accurate results is with the filar or screw micro- 

 meter (Fig. 46). This consists of a metal case to 

 the upper surface of which is fitted an adjustable 

 Ramsden eyepiece. Within is a frame carrying 



