35 
When viewing certain objects by transmitted light, and 
particularly with oblique illumination, a very slight altera- 
tion in the quantity and direction of the light produces a 
marked difference in the appearance of the object, especially 
in Diatomacese, where a proper management of the light 
shows lines or markings invisible under ordinary direct illu- 
mination. 
The apparatus now exhibited is one easily made at a 
trifling cost, and consists of a circular disc of blackened tin 
or cardboard ten or twelve inches in diameter, with a number 
of perforations of various shapes and sizes — circular, cross- 
shaped, wedge-shaped, &c. — the centres of which are about 
inches from the centre on which the disc, placed perpen- 
dicularly, rotates. The form of perforations found generally 
most useful are parallel slits— slits at right angles to each 
other— ^wedge-shaped and circular openings. 
The object under view must be well illuminated in the 
direction required, and then the disc, supported by a pillar, 
is placed between the source of light and the concave mirror, 
when a few trials will determine the best form of aperture. 
The markings of Pleurosigma fasciola, angulatum, &c., may 
be seen by its aid under powers which would not show them 
with any arrangement of achromatic condensers, and it also 
has the good property of shading all but the amount of light 
required from the lower portion of the microscopic stage and 
stand. 
The disc might be attached to the lamp, but it appears to 
work better on a stand, and is susceptible of various modifi- 
cations which will readily suggest themselves to the micro- 
scopist. 
