Reade^ on a New Hemispherical Condenser. 
63 
In the application of this condenser to the resolution of 
lined test objects,, it will be seen that the principle sought to 
he carried out is to throw the axis of the pencil of illumi- 
nating rays in a direction at j:*ight angles to the line to be 
resolved. In all cases where the precise position of such 
lines is known, a supplementary diaphragm may be cut with 
the apertures in their correct mutual positions ; but as these 
position angles greatly vary in diflFerent diatoms on the same 
slide, my friend, Mr. Waterhouse, ingeniously suggests the 
use of a pair of similar diaphragms overlying each other, and 
capable of revolution round a common centre. For this 
purpose the diaphragm next the condenser must be fixed in 
position, and moveable with the lens, by means of the pinion 
motion of the sub-stage, while the other is attached to a deep 
hoop fitted upon the brass tube carrying the lens, so as to be 
conveniently rotated by the finger and thumb, applied to a 
narrow milled ring, but sufficiently small to pass through the 
opening of the second stage, when the condenser is required 
to be removed for other purposes. To carry out this sug- 
gestion, place two diaphragms together, and mark out on, 
their circumference the positions of six adjacent apertures; 
cut out one aperture, pass over two, and cut out the remaining 
three ; then turn them face to face, so that the small stops 
between the apertures may coincide, and, by the rotation of 
one diaphragm upon the other, the stop between two aper-; 
tures, or little prisms, as they virtually are, may be made to 
vary from about 30° to 120°. This will be ample scope for 
all bilinear, oblique, and rectangular markings. This 
method of arrangement also admits of the introduction of 
a third aperture for the P. angulatum, &c., and the whole 
diaphragm system is thus brought within the least possible 
compass. 
The lens in its present form is simple, cheap, and easy of 
adjustment, though of course not free from chromatic aber- 
ration ; but the proper adjustment of the apertures to the 
object examined seems to prevent this error from being very 
apparent, and a pierced diaphragm beneath as well as upon 
the condenser has advantages in this direction, as well as 
occasionally in others. The central pencil of about -^ih. of 
an inch in diameter, which gains intensity from the con- 
struction, is itself virtually achromatic, and is also very 
effective for direct central illumination where obliquity is not 
required, or would be injurious. 
The angle of aperture of the lens is necessarily small ; and 
therefore I cannot help thinking, with Mr. Tomkins, that 
if it were possible by the .application of rotating pierced 
VOL. I. NEW SER. / 
