174 On cm Improved Method of 



separation be effected, indeed, that I have frequently obtained light 

 enough to give distinct vision and admirable definition on the card- 

 board screen with five thousand linear diameters or even higher 

 powers (obtained by the immersion ^(jth, an amphfier, and four 

 feet or greater distance), while the heat was so slight that the drop 

 of water used with the immersion lens did not require renewal 

 oftener than about once in two hom:s. 



I had employed this device for several months, and supposed it 

 to be quite novel, when I read the paper of the late distinguished 

 President of the Eoyal Microscopical Society of London, the Kev. 

 J. B. Keade, " On the Separation of the Eays of Heat from the Kays 

 of Light in Solar and Oxy-hydrogen-gas Microscopes." * I learned 

 from that article that Mr. Eeade had devised this very plan as an 

 improvement to the solar microscope as long ago as 1836. The 

 advantages attained may be stated in his own lucid words : — 



"It Ls evident by this arrangement of lenses we convert the 

 parallel solar beam first of all into a cone of light-giving rays within 

 a cone of heat-giving rays, and the principal focus of heat is farther 

 from the condensing lens than the principal focus of Hght. But 

 after these rays cross the axis we have, conversely, an equal and 

 opposite cone of heat-giving rays within a cone of light-giving rays, 

 and a plano-convex lens or hemisphere, if placed in this second 

 cone at the distance of its own focal length from the principal focus 

 of heat, will be at a distance greater than its focal length from the 

 principal focus of light ; and consequently the rays of heat, after 

 passing through this lens, ^vill become parallel, while the rays of 

 light converge to a second focus. I have approximately measured 

 the heating power of the thermal rays of the second cone when 

 rendered parallel by the plano-convex 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^ Fahr., while at 

 the same time the heat at the focus of the first cone was sufficient 

 to discharge gunpowder." 



Mr. Eeade appears to have experimented with low-power objec- 

 tives only, for he speaks merely of such preparations as the head of 

 a flea. He therefore succeeded very well by using a single lens in 

 his field-glass. With such powers as the immersion -^th and Yi;th 

 I find it better to use an ordinary achromatic condenser instead. 

 The principles involved are of course identical. For the first con- 

 denser also, I have been using an achromatic combination of the 

 dimensions and focal length above mentioned, taken from the back 

 of an ordinary jjhotographic portrait tube, but I am not sure that 

 a simple plano-convex lens of the requisite diameter and focal length 

 would not answer every purpose. 



* ' The Britibh Journal of Pliotograiihy,' December 16, 1870, p. 590. 



