New Paraboloid Illuminator, &c. By James Edmunds, M.D. 81 
also the film of glycerine may be so far thickened or thinned out, as 
to give ample range of focal distance between the reflecting surface 
of the paraboloid and the object. Thus, by very simple manage- 
ment, an object under Powell and Lealand’s twenty-fifth, sixteenth, 
or new immersion eighth of 140°, may be bathed in light until it 
glows with luminous radiance upon a perfectly black background. 
By slight alterations in focussing, or in centring the paraboloid, 
the light upon the object may be made converging or diverging, or 
it may be instantly extinguished. By extremely small inclinations 
of the plane mirror beneath, the paraboloid is made to act as a 
powerful prism, and the object may be bathed with a pure broad 
band of monochrome, from black-red through a brilliant luminous 
green, on to blue, and black-violet. 
Amphipleura pellucida, in this pure white light, shows like a 
transparent three-edged file floating in black space and glowing with 
its own radiance. On slightly inclining the mirror, the diatom 
becomes black-red or disappears. On the tenderest possible further 
alteration of the mirror, the diatom stands out as luminous as before, 
but in splendid green with its cross-markings in black ; as the in- 
clination of the mirror is increased, the diatom becomes pure blue, 
then violet and hazy, and then again invisible. On slowly restor- 
ing the mirror to its original position, the diatom travels backwards 
through the same phases, but its cross-markings only come out well 
between the bright blue and the low green, and they are most dis- 
tinct where the low blue merges into splendid green. Other 
objects such as Navieula rhomboides, which are in themselves 
prismatic, of course add their quota to the grand play of colours 
produced by the prismatic action of the parabolic lens. Such 
objects as give coloured images from illumination by pure white 
light may generally be viewed in red, green, or blue separately, by 
inclining the mirror or by focussing the objective. Taking the 
pure luminous green as the best image, the red comes out on short- 
ening the focus of the objective, the blue on lengthening it. More- 
over, these prismatic images, and the well-known diffraction bands, 
may be developed so splendidly by the paraboloid, that their images 
will probably become as useful for the measurement of extremely 
delicate objects in the field of the microscope, as Newton’s rings 
have proved elsewhere for calculating magnitudes infinitely too 
small for mechanical measurement. 
Bacterial fluids have been the despair of microscopists. Under 
the best modern immersion objectives, many important specimens 
give only a foggy, impenetrable field, in which dim spheroidal 
forms start into view only to fade out again and be lost in fog. If 
the new paraboloid, tipped with glycerine, be inserted into the sub- 
stage, and gently racked up until its glycerine suffuses the under 
surface of the slide and spreads out so as to render the lens and 
