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Transactions of the Society. 
or to a tube of compressed hydrogen, supplies gas to the “ mixed ” jet 
(which has a much smaller nipple than usual) at a pressure of 2 inches 
of water ; so that, with the hydrogen tap full open, the light is always 
of the same power, and exposures may be timed with certainty. The 
oxygen is better taken at a higher pressure direct from a tube 
having only the usual spring regulator, and controlled by the screw 
tap of the jet ; but it is as well that the spring should be a weak one. 
The zirconia cylinder which is used, with the jet directed on to the 
end of it, gives an incandescent point almost as" small as the electric 
arc, and is at the same time much more manageable, and in every 
way superior to lime for the present purpose. With it the beam 
emerging from the first combination is almost perfectly parallel. If 
# a larger radiant such as a lamp-flame or Welsbach burner be used, 
it is much better to bring up the back combination as near to the 
other as the intervening support will allow’. 
It will be noticed that not only is the emergent beam entirely 
robbed of its heat by passing through so large a depth of water, but 
the jet is, in virtue of the whole arrangement, removed to so great a 
distance from the Microscope, that there can be no indirect communi- 
cation of heat, such as might affect the focal adjustment in high 
power work. The illumination is also of such power that very deep 
and approximately monochromatic screens may be used, with moderate 
exposures ; and excellent work may thus easily be done with objec- 
tives which are not apochromatic. 
The device of the parallelising plano-concave lens is of course 
originally due to Kingsley, who first employed it in a Microscope for 
screen projection about the middle of the present century, and it has 
since been used by Mr. Lewis Wright in his oxy hydrogen lantern 
Microscope, now so well known. 
The large iris in front of the combination next the light is an 
important feature. By it the substage condenser is focused and 
centered, and when it is shut down to the right extent, it will be 
found (besides rendering the whole system perfectly aplanatic) to en- 
tirely cut off that surplus light, which when reflected from the inside 
of the Microscope tube and objective mount, causes so much trouble 
and so many failures in photomicrography. So that it is no longer 
necessary to use a specially large tube, and the small tube, with the 
greater sensitiveness it confers upon the fine focal adjustment, can be 
retained. If no light whatever is to strike the sides of the tube 
(though a little does not of course matter) the diaphragm of the sub- 
stage condenser must not be opened beyond its full aplanatic aper- 
ture. 
There is, besides, a small iris beyond the plano-concave parallelising 
lens which cuts off any remaining stray light, and is also used for 
centering. 
The whole system is provided with centering screws, and it, like 
the Microscope, is centered once for all and never again touched. The 
