1885.] on Sunlight and the Earth's Atmosphere. 273 



know what good work it has done, but even that is not sensitive 

 enough to measure in the grating spectrum, in some parts of which 

 the heat is four hundred times weaker than in that of a prism, and we 

 want to observe this invisible heat in very narrow spaces. Something 

 like this has been provided since by Captain Abney's most valuable 

 researches, but these did not at the time go low enough for my 

 purpose, and I spent nearly a year before ascending the mountain in 

 inventing and perfecting the new instrument for measuring these, 

 which I have called the "bolometer" or "ray-measurer." The 

 principle on which it is founded is the same as that employed by my 

 late friend. Sir Wm. Siemens, for measuring temperatures at the bottom 

 of the sea, which is that a smaller electric current flows through a 

 warm wire than through a cold one. 



One great difficulty was to make the conducting wire very thin, 

 and yet continuous, and for this purpose almost endless experiments 

 were made, among other substances pure gold having been obtained 

 by chemical means in a plate so thin that it transmitted a sea-green 

 light through the solid substance of the metal. This proving un- 

 suitable, I learned that iron had been rolled of extraordinary thinness 

 in a contest of skill between some English and American ironmasters, 

 and, procuring some, I found that 15,000 of the iron plates they had 

 rolled, laid one on the other, would make but one English inch. 

 Here is some of it, rolled between the same rolls which turn out plates 

 for an iron-clad, but so thin that, as I let it drop, the iron plate 

 flutters down like a dead leaf. Out of this the first bolometers were 

 made, and I may mention that the cost of these earlier experiments 

 was met from a legacy by the founder of the Royal Institution, Count 

 Eumford. The iron is now replaced by platinum, in wires or rasher 

 tapes from 1-2000 to 1-20, 000th of an inch thick, one of which is 

 within this button, where it is all but invisible, being far finer than a 

 human hair. I will project it on the screen, placing a common small 

 pin beside it as a standard of comparison. This button is placed in 

 this ebonite case, and the thread is moved by this micrometer screw, 

 by which it can ^e set like the spider line of a reticule ; but by means 

 of this cable, connecting it to the galvanometer, this thread acts as 

 though sensitive, like a nerve laid bare to every indication of heat and 

 cold. It is then a sort of sentient thing : what the eye sees as light 

 it feels as heat, and what the eye sees as a narrow band of darkness 

 (the Frauenhofer line) this feels as a narrow belt of cold, so that when 

 moved parallel to itself and the Frauenhofer lines do wn the spectrum 

 it registers their presence. 



It is true we can see these in the visible spectrum, but you 

 remember we propose to explore the invisible also, and since to this 

 the dark is the same as the light, it will feel absorption lines in the 

 infra-red which might remain otherwise unknown. 



I have spent a long time in these preliminary researches ; in 

 indirect methods for determining the absorption of our atmosphere, 

 and in experiments and calculations which I do not detail, but it is so 



Vol. XI. (No. 79.) t 



