60 Keade^ on a New Hemispherical Condenser. 
The hemispliere in question has been many years in my 
possession^ though I did not apply it to the table micro- 
scope until February, 1860. It happened to be one of 
the lenses which Mr. Chamberlain, the optician, called " a 
sporting lot and I may say, that on more than one 
occasion I have successfully used it in following optical 
game. In the year 1837 it did me good service when con- 
nected with the condensing lens of a solar microscope, inas- 
much as it gave me great light with little or no heat, and 
thereby prevented all risk in the use of achromatic object- 
glasses and objects mounted in balsam. 
The arrangement for this purpose is as follows : — A beam 
of solar light, containing both colorific and calorific rays, was 
transmitted through the condensing lens of the instrument ; 
and, owing to the difi'erent refrangibility of these com- 
ponents of the beam, we have a cone of light-giving rays 
formed within a cone of heat-giving rays, and the principal 
focus of heat is further from the lens than the principal 
focus of light. But when these rays cross the axis, the cone 
of heat-giving rays lies within the cone of light-giving 
rays ; and, if the hemispherical lens be placed in these 
second cones, at the distance of its own focal length from the 
principal focus of heat, it will be at a greater distance than its 
focal length from the principal focus of light; and, conse- 
quently, the rays of heat will be rendered parallel, while the 
rays of light will converge to a second focus, exhibiting great 
intensity of illumination, but without any sensible heat. 
I have approximately measured the heating power of the 
calorific rays in the second cone, when rendered parallel 
by the hemispherical lens; and I found, in the month of 
December, that the mercury in a sensitive thermometer, 
when placed in the second focus, did not reach 90° Fah., 
while, at the same time, the heat in the focus of the first 
cone was sufficient to discharge gunpowder. 
The admirable drawing, by Lens Aldous, of the magnified 
head of a flea mounted in balsam, from which his well-known 
lithograph was made, is a good illustration of the practical 
value of this application of the lens ; and it is probable that a 
cemented achromatic object-glass was then, for the first 
time, used with safety in the solar microscope. 
I also used the hemisphere, with a central disc of tinfoil 
upon its plain surface, as a means of obtaining a black- 
ground illumination in the solar microscope; and nothing 
can exceed the beauty of the brilliant tint of the Volvox 
globator and Hydra viridis under this arrangement. I found 
it impossible, however, to take a photograph of these objects 
