Royal MierosGopieal Society. 
57 
annulus, which I shall call the black test-band. If the object-glass 
aperture be reduced, or if another object-glass be used of much 
less aperture, this circular blackness will appear much increased in 
breadth. And upon careful micrometrical measurements being 
made, it will be found for the same aperture that the breadth of the 
hlacJc ring is exactly in the same jprojoortion to the diameters of the 
spherules. Indeed this phenomenon is so striking that the angular 
aperture is at once shown by the breadth of the picture displayed 
within the spherule or spherical lens. 
Upon increasing the aperture the picture becomes larger and 
larger, and more and more indistinct and confused, until with a 
large aperture the ring is attenuated exceedingly ; but of course as 
the aperture is increased the spherules chosen must be smaller in 
proportion to the power of the glass. 
Upon diminishing the aperture exceedingly, the aperture test- 
band widens so much that only a minute picture is left in the 
centre, which can be further diminished to a bright dot. 
This aperture test -band has a remarkable effect upon definition. 
If we are observing very minute spherules in a mass, with excessive 
aperture the aperture bands become almost invisible. The forms 
of closely-packed beading if refractive and transparent cannot be 
descried. Each bead under large aperture-vision forms a confused 
picture ; and if there be brilliant illumination the forms under 
inspection are completely obliterated. 
Experiment 2. — Globule of glass Wo o^h of an inch in diameter. 
Aperture 140°, dry iih, 1862. Aperture band invisible. A stop 
is now placed behind the back set of lenses yioth of an inch in 
diameter ; a large, broad, black annulus is instantly produced. The 
breadth of the aperture band measures the reduction of aperture. 
II. Another principle affects the breadth and distinctness of 
the test-band, viz. the refracting power of the spherule itself. 
This band will not be developed in a spherule of water with a 
greater angle than 60° 16' of angular aperture.* 
A minute spherule of plate glass will begin to show the aperture 
band at so high an angle as 83° ; blue sapphire at 124° 30'. But' 
a heavy glass bead, consisting of two parts lead and one of flint, 
will begin to develope the aperture band at 164°. 
The aperture bands are shown equally well in cylindrical 
threads as in spherules. 
It is evident that, conversely, a coarse measure of the refractive 
index of any proposed substance can be got from the breadth of this 
band, provided the substance can be formed into minute spherules 
or cylindrical threads. There is, here, a nice distinction to be made 
as regards microscopical definition, in the double eifects of variable 
* For the mathemutical consideration of this point, see the article already 
quoted. 
