New Paraboloid Illuminator, &c. By James Edmunds, M.D. 79 
from the optic axis, we get the following results. The ray a is in 
large part reflected from the under surface of the slide, as a 1 , and 
the proportional magnitude of a 1 will increase in a great ratio as 
the obliquity of a itself augments. The reflected ray a 1 falls into 
the spherical hollow of the paraboloid, or upon sub-stage fittings, and 
in considerable part is reflected back into the slide at low angles. 
a 1 , therefore, is not merely lost, it is detrimental by fogging 
the illumination, a 2, on entering the slide, undergoes refraction, 
and, assuming a low index for the slide (1 - 5), it will be seen by a 
simple geometrical construction or by reference to the table of sines 
that it reaches the upper surface of the slide with its obliquity 
reduced to about 35°. Its achromatism also will be damaged in 
proportion to the thickness of the slide. At the upper surface of 
the slide a 2 undergoes a second division into a 3 and a 4, a 3 
being reflected back in the slide to its lower surface, where it splits 
into b and b 1. b 1 passes out below, falls upon the sub- stage fit- 
tings, and, in part, returns into the slide at low angles to add to 
the detrimental effects of a 1. b goes on repeating the results 
produced by a 2, and at each cycle in its course, throws a ghost 
image into the field. 
Eeturning now to the remainder of the original pencil a 4, 
it will be seen that it passes out through the upper surface of the 
slide ; and the ray, thus reduced in obliquity, weakened in power, 
and damaged in colour, enters any object which is optically con- 
tinuous with the slide. Otherwise the ray undergoes a second 
refraction, and strikes the object or cover-glass at the original angle 
of 60°. Thus in any case the ray which enters the object is enor- 
mously weakened in power, and is more or less chromatized, while 
the illumination of the field is damaged with fog and with ghost 
images. And if the obliquity of the original pencil be made 
greater than 60°, the drawbacks all increase in a great ratio. 
While working at difficult objects with high powers, it seemed 
to me that all these disadvantages might be got rid of, and our 
command of oblique sub-stage illumination made perfect by the new 
paraboloid of which I now submit a description, and which, having 
been constructed for me by Messrs. Powell and Lealand, has fully 
answered the expectations which were raised by optical and geo- 
metrical considerations. 
A paraboloid lens of glass, free from veins and from colour, 
and of the lowest obtainable refractive index, is made. The apex 
of the lens is cut off at a point, one-twelfth of an inch below its 
latus rectum or internal focus. The whole surface is brought to 
an optical polish. The lens is set with its base left clear as far 
as possible, and with a flat shoulder projecting about one- fiftieth 
of an inch below its face, so that diaphragms may be adjusted 
to work against this shoulder, close beneath the base of the lens, 
