MICROSCOPIC ILLUSTRATIONS. 



291 



The angle of Aperture of an object-glass is very difficult to 

 be exactly found, on account of its thickness, and still more 

 so, if several are combined together, by the methods already 

 given; but Mr. Lister has discovered a very exact and 

 satisfactory method of doing this, which he has given in 

 the paper already alluded to, p. 191. The principle of it 

 may be seen in figure 18. B represents the object-glass 

 of an engiscope placed in a horizontal position, and C F 

 its axis, directed so far on one side of a candle A, placed 

 at a few yards distant, that the light of it shall bisect the 

 field of view vertically, leaving half of it dark : then C 

 being the focus of the object-glass, is to form a pivot, round 



Fig. 18. 



