80 Transactions of the Royal Microscopical Society. 
without abrading its surface. By an arrangement which it would 
be difficult to make clear without diagrams, any combination from a 
set of central disks, peripheral zones, and revolving shutters can be 
so placed beneath the lens, as to admit light in any or every azi- 
muth, and at any or every angle, and the light may come through 
a stage of any depth, from a simple plane mirror. In short, a 
splendid pencil of pure unrefracted light may be easily converged 
upon an object placed at the focus of the lens, if only the object be 
made optically continuous with it by means of a fluid or cement 
which is optically homogeneous with the lens. 
If the lens be adjusted with a central disk stopping out from 
its base only such light as would pass directly through the upper 
plane surface, and sunlight be thrown up, the action of the lens is 
demonstrated with great splendour. The plane top throws back all 
the light by total reflection, and remains itself perfectly black, when 
viewed from above. But, if there be dust resting upon the plane, 
each particle becomes brilliantly luminous where the light from 
beneath impinges, and if, for the purpose of demonstration, the top 
be smeared with wax, or have tissue paper gummed on to it, there 
is seen a brilliant circle of light surrounding a sharply defined 
central black disk, about a fourth of an inch in diameter. 
On placing upon the top of the lens a plate-glass slide one- 
sixteenth of an inch thick — fine tissue paper having been gummed 
on to the upper surface, and a drop of glycerine used below to 
cement the slide to the paraboloid — a central focus of intense white 
light is seen on the tissue paper. On lowering the paraboloid, but 
not sufficiently to separate the film of glycerine, this brilliant focus 
spreads into a small circle with a central black spot — the tissue 
paper having been raised above the focal point. If in place of the 
tissue paper there be cemented on to the slide a hemisphere of 
glass, and, by the stage movements, the centre of the plane surface 
of the hemisphere be located at the focus of the paraboloid, then all 
the light which reaches the focus diverges again and passes out 
through the hemispherical surface at a few degrees above the 
horizon without refraction. Here, while the eye in every azimuth 
at that angle receives a brilliant picture of the sun, no light what- 
ever is received by the upper portion of the hemisphere. If the 
optical continuity between the slide and the paraboloid be broken 
by lowering the paraboloid until the film of glycerine be separated, 
the image of the sun instantly disappears, all the light being then 
thrown back out of the base of the paraboloid. 
I find that the glycerine film makes the slide optically con- 
tinuous with the top of the paraboloid, and at the same time acts as 
a perfect lubricant, allowing the slide to travel over its surface in 
any direction as if it were perfectly free, and without damaging its 
optical continuity with the lens. By lowering or raising the lens 
