HOW SHALL WE MEASURE ANGULAR APERTURE. 95 



HOW SHALL WE MEASURE ANGULAR APERTURE. 



While the nature and functions of angular aperture 

 remain a matter of controversy, this too may be allowed 

 to remain an open question. Recognizing, however, 

 the fact that object-glasses continue to be made and 

 sold, the makers claiming for them specific apertures, 

 we proceed to give the plan usually adopted by the 

 optician, and at the same time adapted to the use of 

 those working with the smaller class of stands. 



It will be advantageous to the beginner to operate 

 either in a dark room or in the evening. Select a good 

 sized table, and on this, at the end nearest the operator, 

 pin down a large sheet of paper, say twenty inches 

 square. On this paper place the microscope, bring the 

 tube to a horizontal position, and screw on the object- 

 ive to be tested. Mark with a pencil certain points 

 along the base of the stand, so as to enable you to 

 describe a circle with pencil and dividers, in which the 

 base will just revolve without lateral play. Next, 

 provide a common candle, and, if necessary, cut this 

 down so that the height of the flame shall be level with 

 the objective in its horizontal position ; light the candle, 

 point the objective directly at the light, having pre- 

 viously removed all sub-stage incumbrances, crouching 

 down, and with the eye at the eye-piece, adjust the 

 direction of the instrument as nearly as possi ble to the 

 candle ; the field of view will of course be illuminated 

 from the radiant. 



Now revolve the entire instrument, being careful to 



