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Transactions of 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. 
“ Remembering 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 he 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.” .... “Remarkable 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 effect 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 fths to T ^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 light. 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 black 
* ‘Quart. Journ. Micro. Sci.,’ Jan., 1870. 
t It is best to begin witli a low aperture objective. 
