May 29, 1885.] 



SCIENCE 



445 



on the screen, without the sun itself; but here are 

 photographs of it, which show parts of the losses the 

 different colors have suffered on their way to us. 

 We have before us the well-known Fraunhofer lines, 

 due, you remember, not only to absorption in the 

 sun's atmosphere, but also to absorption in our own. 

 We have been used to think of them in connection 

 with their cause, one being due to the absorption of 

 iron-vapor in the sun, another to that of water- 

 vapor in our own air, and so forth ; but now I ask 

 you to think of them only in connection with the fact 

 that each is due to the absorption of some part of the 

 original light, and that collectively they tell much of 

 the story of what has happened to that light on its 

 way down to us. Observe, for instance, how much 

 thicker they lie in the blue end than in the red, — an- 

 other evidence of the great proportionate loss in the 

 blue. 



If we could restore all the lost light in these lines, 

 we should get back partly to the original condition 

 of things at the very fount; and, so far as our own 

 air is concerned, that is what we are to ascend the 

 mountain for, — to see, by going up through nearly 

 half of the atmosphere, what the rate of loss is in 

 each ray by actual trial; then, knowing this rate, to 

 be able to allow for the loss in the other part still 

 above the mountain-top ; and, finally, by recombining 

 these rays, to get the loss as a whole. Remember, 

 however, always, that the most important part of the 

 solar energy is in the dark spectrum, which we do not 

 see, but which, if we could see, we should probably 

 find to have numerous absorption-spaces in it corre- 

 sponding to the Fraunhofer lines, but where heat has 

 been stopped out rather than light. To make our 

 research thorough, then, we ought not to trust to 

 the eye only, or even chiefly, but have some way of 

 investigating the whole spectrum, — the invisible, in 

 which the sun's power chiefly lies, as well as the visi- 

 ble, and both with an instrument that would dis- 

 criminate the energy in these very narrow spaces like 

 an eye to see in the dark; and, if science possesses 

 no such instrument, then it may be necessary to in- 

 vent one. 



The linear thermopile is nearest to it of any, and 

 we all here 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 provid- 

 ed since by Capt. 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 ascend- 

 ing the mountain, in inventing and perfecting the 

 new instrument for measuring these, which I have 

 called the ' bolometer,' or ' ray-measurer.' The prin- 

 ciple on which it is founded is the same as that 

 employed by my late lamented friend, Sir William 

 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 ob- 

 tained by chemical means in a plate so thin that it 

 transmitted a sea-green light through the solid sub- 

 stance of the metal. This proving unsuitable, I 

 learned that iron had been rolled of extraordinary 7 

 thinness in a contest of skill between some English 

 and American iron-masters; and, procuring some, 

 I found that fifteen thousand 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 flut- 

 ters down like a dead leaf. Out of this the first bo- 

 lometers 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 Rum- 

 ford. The iron is now replaced by platinum, in wires, 

 or rather tapes, from a two-thousandth to a twenty- 

 thousandth 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 be 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 indi- 

 cation of heat and cold. It is, then, a sort of sen- 

 tient thing: what the eye sees as light it feels as heat, 

 and what the eye sees as a narrow band of darkness 

 (the Fraunhofer line) this feels as a narrow belt of 

 cold; so that, when moved parallel to itself and the 

 Fraunhofer lines down 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 invisi- 

 ble 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 re- 

 searches, in indirect methods for determining the 

 absorption of our atmosphere, and in experiments 

 and calculations which I do not detail ; but it is so 

 often supposed that scientific investigation is a sort 

 of happy guessing, and so little is realized of the 

 labor of preparation and proof, that I have been 

 somewhat particular in describing the essential parts 

 of the apparatus finally employed ; and now we must 

 pass to the scene of their use. 



We have been compared to creatures living at the 

 bottom of the sea, who frame their deceptive tradi- 

 tional notions of what the sun is like from the feeble, 

 changed rays which sift down to them. Though such 

 creatures could not rise to the surface, they might 

 swim up towards it; and if these rays grew hotter, 

 brighter, and bluer, as they ascended, it would be al- 

 most within the capacity of a fish's mind to guess that 

 they are still brighter and bluer at the top. Since 

 we children of the earth, while dwelling on it, are 

 always at the bottom of a sea, though of another sort, 

 the most direct method of proof I spoke of, is merely 



