New Form of Photomicro graphic Camera. By E. B. Stringer. 177 
lateral tubes and those on the bellows supports are adjusted to a con- 
venient tightness, and are not afterwards touched. 
The condensing system, the oxyhydrogen jet, and a small lantern 
of sheet iron, are all carried on the optical bench, which I have found 
it a great improvement to make square in section, instead of prismatic 
as has been usual heretofore. With the condensing system many dif- 
ficulties were at first experienced, until Mr. E. M. Nelson very kindly 
interested himself in the matter, and computed a new set of conden- 
sers, following the same general plan as I had done, but making them 
larger (4J inches in diameter) and employing the new Jena glasses. 
These, after one or two final alterations, proved entirely successful, 
and the result (the perfection of which is due to Mr. Nelson’s optical 
knowledge and skill) is an illuminating system perfectly achromatised, 
and almost perfectly aplanatic, and moreover of much greater power 
than has hitherto been available, affording a beam of great intensity, 
and of that slight divergence which is best suited to the working of 
the substage condenser. 
The working of the whole system is as follows : — The light from 
the jet is first parallelised by the doublet condenser, which consists of 
two plano-convex lenses having their plane sides turned towards the 
radiant, the one next the radiant being a quarter of an inch less in 
diameter than the other, and the focal length of the combination 
being 2f inches, taking up an angle of 70°. 
The parallel beam passes across an interval of about 10 inches, 
through the screen or trough of coloured solution carried by the inter- 
vening support, and enters the plano-convex lens 4^ inches in diameter 
(having its convexity turned the other way to minimise the aberra- 
tions), by which it is converted into a converging cone. This, after 
passing through the water in the chamber between the lenses, is 
again parallelised by the much smaller plano-concave lens, and emerges 
from it as a parallel, or more strictly speaking a slightly divergent 
pencil, rather less than an inch in diameter, and enters the substage 
condenser. The plano-concave lens is of highly dispersive glass, and 
perfectly achromatises the whole system. 
The converging arrangement is clamped at such a distance from 
the substage condenser as to throw a spot of light rather larger than 
the opening in the condenser diaphragm. The smaller the radiant 
point, or the larger the opening in the diaphragm, the greater this 
distance should be. The best average distance is about 2 J inches from 
the substage ring as it stands at present. This also allows ample 
room for putting down and taking up the Microscope, and for using 
the mirror when the Microscope is in place, if it should be necessary 
to do so. For “dark ground” work, or in using the Lieberkuhn, 
the small iris may be removed, and the nose of the converging system 
slid right up into the substage ring, so that there is no loss of light 
whatever. 
The small weighted cone regulator, attached either to the main 
