4 THE AMERICAN MONTHLY [January, 



to determine the value of the intervals between the lines with each ob- 

 jective which it is intended to use with the eye-piece. This operation 

 having been performed, the stage micrometer can then be dispensed with. 

 In order to make this determination, a stage micrometer ruled to, say 

 i-ioo" and i-iooo", is laid upon the stage of the microscope and brought 

 into focus. The lines of the stage micrometer are now brought par- 

 allel to one of the series in the eye-piece, and one of the lines of this 

 series made to coincide with one of the lines of the stage micrometer 

 by shifting the latter till the desired position is arrived at. It must now be 

 observed how many of the divisions of the stage micrometer correspond 

 to one division of the eye-piece micrometer, and it is well to pull out 

 the draw-tube till some simple relation is arrived at. Thus, supposing 

 we have taken a stage micrometer divided into i-ioo" and i-iooo", and 

 we find that one of the larger divisions of the stage micrometer (equal 

 to i-ioo") corresponds to 9^ divisions of our ruled glass disc in the eye- 

 piece, we should pull out the draw-tube till it corresponded exactly to 

 ten divisions, and each division would now correspond to one of the 

 smaller divisions of the stage micrometer, or i-iooo". The distance to 

 which it is necessary to pull out the draw-tube must then be noted down 

 for future reference, as also the fraction of an inch corresponding to 

 each division of the eye-piece micrometer. 



This operation must naturally be repeated for each objective. It is 

 well to perform the operation several times with each objective, using 

 different portions of the two micrometers, and taking the mean of the 

 results, as, although lines can now be ruled on glass at very close inter- 

 vals with a high degree of accuracy, it is always well to correct, as far 

 as possible, any slight error there may be in the ruling of the lines on 

 your micrometer, or arising from errors of observation. It is also 

 necessary, when using these objectives which are provided with the 

 means of adjustment to the thickness of the glass that covers the object, 

 to make an allowance for this, since the magnifying power is consid- 

 erably affected by the separation of the lenses. It will be found con- 

 venient to compensate for this alteration by altering the draw-tube in 

 such a manner as to neutralize the effect produced by the adjustment of 

 the objective, thus giving one vmiform value to the divisions of the eye- 

 piece, whatever may be the thickness of the cover-glass. 



The amount of alteration must, of course, be determined by a series 

 of measurements with the stage micrometer, and noted at the time. 



Suppose now we have, on the stage of the microscope, any object 

 which it is desirable to measure, and are using an objective with which 

 it has been found that one division of the eye-piece micrometer= 

 i-iooo'\ the draw-tube being pulled out to the proper distance, as noted 

 at the time when the determination of the values of the eye-piece mi- 

 crometer divisions was made, the object is brought into such a posi- 

 tion that one of its edges seems to touch one of the lines of the microm- 

 eter, while the diameter which is to be measured lies at right angles to 

 the micrometer lines. It is then observed where the opposite edge 

 falls, and what is the number of divisions between the two along the 

 diameter. Suppose the object occupies 5^ divisions of the micrometer ; 

 it will then measure 5^-1000" or 0.0055". 



The next simplest form of micrometer consists of a transparent glass 

 scale, which is introduced through a couple of slots in the eye-piece, 



