144 The Microscope. 



certain the aperture of a microscope objective, the measurement, 

 should be made with the lens in actual use, as it was intended 

 to be used, and not as a spy glass or telescope, yet it is the tele- 

 scopic method that is most frequently described and recom- 

 mended in books on the microscope. 



The method that I demonstrated at the meeting of the Ameri- 

 can Society of Microscopists at BufiFalo, and shall here describe^ 

 is that used by the late Robert B. Tolles and of which he is, as- 

 far as I know, the originator. It is essentially as follows : 



The objective to be measured is attached to the microscope- 

 with an eye-piece exactly as for ordinary work, and is focussed 

 on some suitable object in the centre of the field, the correction 

 collar being used if necessary to get ihe best image the objective 

 is capable of giving. The object should be a transparent one, 

 and one the resolution of which is a fair test of the powers of 

 the lens. After these arrangements have been completed, the 

 body of the microscope is turned to a horizontal position, the 

 mirror swung out of the way, and the object illuminated from 

 below the stage by a narrow radiant, such as the flame of a toy 

 candle. The source of illumination is then moved to the right, 

 and the left in succession till either the centre of the field be- 

 comes darkened or the image is spoiled. The angular value of 

 the distance through which the souice of illumination can be 

 moved before this takes place is the available angular aperture- 

 of the lens ; that is, the useful aperture for definition. In some 

 lenses the image is spoiled long before the centre of the field is 

 darkened, so that the aperture, for mere transmission of light, is 

 much greater than that for definition. Such lenses are imper- 

 fectly corrected for the marginal rays, and their performance can 

 be improved by cutting off these aberrant marginal rays by 

 means of stops or diaphragms, and so reducing their augular 

 aperture (for transmission). 



The plan of measurement here described, if used without any 

 device below the stage, can measure only apertures below l.OQ' 

 Numerical Aperture ; that is to say, angular apertures of 180"* 

 air, of 97° 31' water, or of 82° 17' in glass or homogeneous im- 

 mersion fluid, and must always give the results in terms of the 

 equivalent air angle. Hence when we have occasion to measure 

 the aperture of a lens exceeding, or indeed closely approaching 

 1.00 N. A., it becomes necessary to use, below* the slide, some 



