400 Progress in Science. (July, 
of allowing the slide to be thrown out of the centre to the extent required. 
This new turn-table combines all the advantages of the old construction 
with those peculiar to its original self-centreing form, and is also provided 
with a convenient hand-rest, which will be found a great assistance in forming 
cells with varnish, as it enables the brush to be held with very great steadiness. 
Mr. F. H. Wenham has produced a }th objective capable of working, both 
as a dry and immersion lens, with the same front. Its performance under 
both conditions leaves nothing to be desired. 
An account of ‘‘ An Improved Reflex Illuminator for the Highest Powers of 
the Microscope”? is communicated to the Royal Microscopical Society by 
Mr. F. H. Wenham.* It consists of a cylinder of glass (a) } an inch long 
and ,4,ths of an inch diameter, 
FIG. 2. the lower surface of which is 
worked convex to a radius of 
ssths of an inch. Starting 
from the bottom edge, the 
cylinder is worked off to a 
polished face at an angle of 
64°: close beneath the cylinder 
is set a plano-convex lens of 
1} focus. Parallel rays sent 
through the lens, after leaving 
the lower convex surface of the 
cylinder, would be refracted 
to the point shown by the 
dotted lines if continued in 
solid glass, but by impinging 
on the inclined polished sur- 
face (which is far within the 
angle of total reflection) they 
are thrown on the flat seg- 
mental top; here they would 
be totally refleted and beaten 
down again to the point (d) 
outside the cylinder; but if 
an objec slide (c) be laid over 
the flat top with an intervening 
film of water, the rays proceed 
on till the focal point reaches 
the upper surface, or is slightly 
beyond it; here total reflection now takes place; all the light is concentrated 
to a minute spot in the centre of the field of view of the microscope, and 
most of the rays are available for any obje& brought there by traversing the 
slides over the water top of the illuminator, which must be kept full without 
allowing any to run down the reflecting surface. This illuminator is fitted into 
the ordinary sub-stage, and has an independent rotatory movement of its own 
like that of Nachet’s prism. The cylinder is to be brought up nearly level 
with the stage. The centre of rotation is set true by a dot on the fitting seen 
with a 1} objective. A drop of water is then placed on the top, upon which 
the slide is laid. The required obje@s on the slide are found with a low 
power, and may be distinguished by their brilliant appearance, while those on 
the cover are nearly invisible. The light is thrown up either by the plane or 
concave mirror. The former is generally the best and most controllable; the 
lamp should not be placed much beyond the stage, else its direct rays will get 
underneath and mar the blackness of the field. Having got the best effect 
by tilting the mirror, the illuminator is to be rotated, by which means various 
effets are brought out. The paper reviews the author’s past contrivances 
for the same purpose, and anticipates the objeGions that may be raised against 
* Monthly Micro. Journ., vol. vii., p. 237. 
