62 ReadEj on a New Hemispherical Condenser. 
tion for illuminating any rectangularly marked valve whose 
position on the stage of the microscope is known. One 
point of light must lie over the end of the valve for bringing 
out the horizontal lines; the other will be opposite the 
side of the valve^, and will act on the longitudinal lines ; and 
resolution into dots or squares will be immediately effected 
by adjusting the distance of the condenser. 
For oblique or diagonal markings, the apertures at the cir- 
cumference of the diaphragm must no longer be placed at 90° 
apart, but at such an angle as is indicated by the markings 
themselves. In the case of the P. angulatum, where there are 
three lines of markings, there must be three apertures, 
since with two apertures only, we should exhibit, according 
to their position, any two, and but two, of these three lines, in 
turn, and, at the same time, give a sort of unnatural elongation 
to the peculiar markings on the valve. The size of the 
apertures is 24° at the circumference and opposite side, and 
Y^ths of an inch in the direction of the radius. The 
latter dimension must be less in diaphragms for smaller 
hemispheres, and must never exceed half the radius of the 
condenser. 
In order to secure the best effect, the distance between 
the apertures must be adjusted with considerable accuracy. 
For this purpose I use a diaphragm of thin brass, or of strong 
tinfoil, having one aperture only, and by its rotation under a 
given valve of the P. angulatunij for instance, I bring into 
view the three lines of markings in succession, first the 
horizontal lines, and then the oblique lines, by rotating 
the diaphragm to the right and left, and thus the three 
points at which the apertures are to be made can be deter- 
mined with the utmost precision. If the aperture for the 
horizontal lines be made at the distance of 180° from the 
place thus obtained, these lines will be illuminated on their 
opposite sides, and the three apertures will be 120° apart, as 
in the diaphragm first cut out for me by Mr. Water house, 
who happened to be working with me at the time. But 
in practice I find it not only better, but indispensable, to 
illuminate all the markings on the same side, as by the first 
method, and preserve thereby that uniform direction of the 
shadows which is the key to accurate definition. A set of 
diaphragms thus obtained, and a diaphragm with a minute 
circular aperture in the centre only, for the central adjustment 
of the lens, complete the furniture of the condenser ; and a 
brass ring sliding outside the top of the cylinder on which the 
condenser is screwed conveniently holds the diaphragms in 
their place, and admits of their being readily changed. 
