H. F. ANGUS ON THE APERT03IETER AND ITS USE. 213 



(3) The size of the stop used in the condenser should be 

 accurately determined for exactly those conditions under which it 

 will be used, the difference of a milUmetre in the diameter either 

 way affecting the result, either as regards illumination of object 

 or blackness of field. 



(4) The correct diameter of stop for any given combination of 

 objective, condenser, etc., can be calculated on the basis of the 

 actual aperture of the iris, which cuts down the condenser to 

 that point where it just fills the back lens of the objective and 

 no more (the eyepiece is removed to determine this point). 



(5) The diameter of the actual aperture of the iris (when the 

 condition set forth in Rule 4 is fulfilled), plus a fractional part, 

 which varies inversely with the perfection of the correction of 

 the condenser, will be the diameter of the stop required : the 

 fractional part to be added must be found experimentally for each 

 type of condenser, but when found is a constant for all stops used 

 with any given condenser. For instance, with the Abbe chromatic 

 condenser N.A. 1"20, one-half must be added — that is to say, if the 

 actual iris aperture which just fills the back lens of an objective 

 is 8 mm., then the stop must be 12 mm. in diameter ; if 10 mm., 

 then 15 mm. would be the diameter of the stop required. 



To apply these rules in practice a substage-scale apertometer 

 will be found of constant use. For instance, it is required to cut 

 down an objective |-in., of say N.A. 0*45, so as to give a good 

 dark-ground illumination with an Abbe chromatic condenser 

 N.A. 1*20: the value of the millimetre divisions of the disc are 

 with this condenser, as we have already found, N.A. 0*04 ; then 

 we must insert a stop behind the objective of such size that it 

 will limit the field seen at the back of the objective without the 

 eyepiece to 8 mm., that is N.A. 0*32 (Rule 1). 



Having cut down the objective to a suitable aperture, it is 

 necessary to determine the size of the substage stop ; this can be 

 done without the disc by picking up the microscope and measuring 

 the iris aperture with a pair of dividers, but it can be more 

 conveniently done with the disc, as, owing to the scale being in 



