10 
: ON son, ie 
Hence, 6 or ZNFH = 41°. 48’ nearly. 
2 NFH = 838° . 36! nearly.! 
If L represent the front lens of the objective, it appears 
that the sphericle of glass (when w = 3) cannot transmit a 
greater divergent pencil than 83° 36’. That all objective 
aperture above this is useless for sphericular definition. That 
as the aperture of L is diminished by a stop at KK, the trans- 
mitted pencil of rays is contracted to EDHFK, and a black 
shadow of obliterated rays is developed, represented by the 
segment HNCD, the visual angle of which is NFH. That 
as KK is diminished the black annuli broadens. That this 
effect is independent of the size of the globule, as shown 
at BBB. 
Black outlines of cylindrical refracting bodies follow the 
same law, the breadth of black outline being determined by 
the smallness of the objective aperture. And a system of 
such bodies, whether distributed in cylinders or beads, 
present similar phenomena. 
pr’ | 2 Seal) 
1 For any other refractive body, sin NFH = 7 + ESOT | rs r 
for the sine of the semi-limiting aperture. 
