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Transactions o f the 
taken to choose them lying near the surface, and as small as 
possible. The writer published an article in January, 1870, from 
which the following passages are now selected. 
" Kemembering that when a pencil of parallel rays passes 
through a denser into a rarer medium (supposing common air were 
enclosed), the focal point for a refractive index 1 • 5 would be found 
to lie on the posterior surface of the minute hollow sphericle, i. e. 
on the surface farthest from the eye of the observer. If the con- 
tained gas or air were much attenuated it would approach the 
centre. . . . The field of view presented is independent of the 
aperture of the objective, whilst change of aperture has a very sur- 
prising effect upon the visible characters of the solid glass spherule. 
This change, so decided and important in minute research, has been 
a cause of much surprise and pleasure to the writer, as it appears to 
open a new mode of changing and selecting definition under novel 
conditions." .... "Eemarkable is the result that large aperture 
destroys the black ring-outline of refracting spherules and black 
borders of cylindrical fibres. In innumerable instances the only 
possibility of distinguishing the molecules- of organized particles 
depends upon shadow. ... A fundamental defect of excessive 
aperture is the disappearance of these invaluable characters of 
minute spherules and of fibres capable of refracting light." 
" In some cases, therefore, they appear jet black with a small 
aperture, but most frequently invisible with an excess of aper- 
ture."* 
The appearances of Mr. Slack's invaluable silica films most 
opportunely illustrate the ejBEect of aperture. He has observed 
quite independently that the most minute beading visible with a 
glass of low aperture vanished under increased aperture. 
Now if aperture must be diminished in order to develope the 
black test-band, it is evident that excess of aperture may destroy 
it, so that in the case of direct illumination by parallel rays the 
blackness and breadth of the test-band may wholly depend upon 
the two conditions already stated. 
I have used in these experiments an " iris diaphragm " (con- 
structed for me by Messrs. Beck in 1869); by this instrument the 
aperture can be instantly reduced from f ths to yii^th inch. 
Experiment 1. — Select very fine threads of glass, and, holding 
them 'like an open fan, rapidly pass the ends through the blue edge 
of the steady flame of a wax hght. On examination with the 
microscope t under parallel rays from the plane mirror before the 
window in a good light, fused spherules of glass will be seen, of 
various sizes and degrees of spherical perfection, in each of which 
a minute image of the window appears surrounded by a hiack 
* 'Quart. Journ. Micro. Sci.,' Jan., 1870. 
t It is best to begin with a low aperture objective. 
