212 H. F. ANGUS ON THE APERTOMETER AND ITS USE. 



w 



the disc is inserted below the condenser, the ej^epiece removed, and 

 the number of divisions of the scale visible in the field duly noted. 

 As in the case of the Protractor apertometer, it will usually be 

 necessary to magnify the image so obtained in order to read the 

 scale with accuracy. When set up in this manner, the scale will 

 appear sharply defined right up to the edge of the field, provided 

 that the aperture of the lens measured does not exceed the 

 aplanatic aperture of the condenser. Let us take as our objective 

 of known aperture a ^-in. of N.A. 0*34, and as our condenser the 

 Abbe chromatic pattern N.A. 1*20 (this is the total aperture, 

 the aplanatic aperture is of course very much less, approximately 

 N.A. 0*50). Then, proceeding as above, we shall find that 8| 

 divisions are visible in the field, and that consequently 1 mm. of 

 the scale with this condenser has a value of N.A. 0*04 ; if now 

 we take another objective, say a |-in., and find 5 divisions visible 

 in the field, then we know that the aperture of the lens under 

 examination is N.A. 0*20. It is, of course, obvious that the 

 value of the scale varies with the power of the condenser with 

 which it is used. 



Having described the two instruments and the method of using 

 them, thus disposing of the first objection to an apertometer, 

 viz., its high price, we will now turn our attention to the supposed 

 limited application of such an instrument. I shall confine my 

 remarks to dark-ground illumination, but other applications will 

 no doubt occur to the practical microscopist. 



In the first place let me define exactly those conditions under 

 which a good dark-ground illumination can be obtained. These 

 rules were formulated by the aid of the instrument (substage- 

 scale apertometer) just described, and are capable of verification 

 by any one possessing a similar instrument. 



(1) The aperture of the objective employed must not exceed |, 

 or at the most ^, of the total aperture of the condenser. 



(2) A good dark-ground illumination can be obtained with high 

 powers, if stopped down in accordance with the above rule. 



