6Q ReadEj on a New Hemispherical Condenser. 



Tlie hemisphere 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 different refrangibility of these com- 

 ponents of the beam, we have a cone of light-giving rays 

 formed within a cone of heat-ginng 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 beavity 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 



