Beade, on a Neiv Hemispherical Condenser. 61 
under this illumination, though with direct solar light I 
had no difficulty whatever. 
It is probable that a similar application of the hemisphe- 
rical lens and central stop to the oxyhydrogen microscope, 
which our variable climate almost compels us to use, would 
in like manner throw into the pictures on the screen the 
additional charm of natural colours, and thereby greatly en- 
hance the interest of the exhibition. 
Notwithstanding my use of the condenser in the experi- 
ments just described, it did not occur to me to extend 
the application of it, until I was, as it were, driven by 
necessity. My old parishioners and other kind friends pre- 
sented me with a valuable microscope at the commencement 
of last year ; and not having, in the first instance, any of 
the well-known condensers at hand, I used the light of two 
lamps placed at right angles to each other, and by means of 
suitable lenses I threw sufficient light on the rectangular 
markings of the P. acuminatum and other similar tests. I 
was much pleased with the effect of this simple method of 
illumination ; and I am glad to find that Mr. Tomkins has 
also used it, but with considerable improvement, by employ- 
ing two achromatic prisms, which give brilliant illumination, 
while the marking shadows" are in deep relief. 
In order to obtain any proper definition of the markings, 
I found it necessary so to' turn the valve of the diatom, that 
a line of markings might lie at right angles to a line of light. 
In fact, in any other position the markings are scarcely 
visible j and the conclusion seemed forced upon me, that the 
ordinary spot lens contains in its circle of light a large 
portion of unnecessary, if not injurious, illumination. With 
this impression on my mind, it suddenly occurred to me, that 
my old friend, the kettle-drum,'^ as Mr. Gravatt calls my 
condenser, might play an important part, if its plain surface 
were covered with tinfoil suitably pierced at the circum- 
fereuce for the tranmission of two pencils of light at right 
augles to each other. I made the experiment, and happily 
I can fall back upon the testimony of well-qualified ob- 
servers as to the success which attended it. The direct illu- 
mination of only one lamp was now sufficient, and, instead 
of rotating the object — always a difficult process in the 
absence of suitable adjustments — it was easier to rotate the 
secondary stage which held the condenser, and so gain the 
proper position of the two points of light. It may be well 
to state, that by taking out the eye-piece, and looking at the 
points of light down the body of the tube, we may at once, 
by the rotation of the sub -stage, place them in the right posi- 
