84 Transactions of the Royal Microscopical Society. 
beyond this angle be required, a third lens may be constructed with 
its apex cut off just below the latus rectum, and, upon the face of 
such a lens, a particular object may be set in glycerine or balsam, 
or may be merely attached to the surface. By stopping out light 
from the base with a central disk equal in size to the top plane, 
plus a peripheral zone, wide enough to stop out all rays which will 
not reach the focus at a sufficient angle, it is clear that an object 
may be drenched with unrefracted light converging upon it at 
practically 90°, and from as large an arc of its own horizon as 
may be determined by the use or disuse of the revolving shutters 
beneath. By cutting off the apex between the latus rectum and 
the vertex, then turning out through the top plane a lenticular 
cavity — its surface being a portion of a small sphere, and its centre 
of curvature located at the focus of the paraboloid — pure unrefracted 
light, considerably beyond the angle of 90° even, maybe converged 
upon an object set near to the focus. The cavity may be occupied 
by gelatine, balsam, water, castor oil, or other medium in which it 
is desired to set the object, the refractive index of the medium being 
of no consequence in this construction ; and a very high power 
might be brought to bear upon an object carefully set for a special 
investigation if covered with a small disk of thin glass. Or an 
object uncovered, or dry, might be supported near the focus of the 
paraboloid by a loose wad of curled hair or other material, the 
image of which would not blend with that of a delicate object. 
Possibly, by such a paraboloid, the illumination of an object may 
be made to reach the point where it could be taken up from above 
the stage by a fine parabolic Lieberkuhn or other reflecting appen- 
dage to the objective. Four such lenses would form an exhaustive 
series, and only one set of diaphragms and sub-stage tubing would 
be needed. 
It is important that the glass of the lens be of very low refrac- 
tive index, and that the optical media between its top plane and the 
object be as nearly as possible of the same refractive index as the 
lens — otherwise in using light at nearly 90° from the optic axis of 
the microscope slight differences in refractive power between the 
paraboloid, the cementing fluid, and the slide will come into play at 
the junction surfaces and deteriorate the illumination. 
I fear lest in submitting this description I have occupied too 
much space. But I find, practically, that these new paraboloids go 
as far beyond the Wenham paraboloid as the Wenham paraboloid 
goes beyond the spot lens. In amount of light, in purity of 
colour, in freedom from fog and from ghost images, in range of 
azimuth and of angle, and in mechanical adaptability to every 
microscope, they are unexceptionable. They fill a gap in our 
illuminating appliances which, of late years, has been increasingly 
felt, and they lift the microscopic objective to a new level. They 
