Page:Science vol. 5.djvu/481

 May -JS, 1885.]

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 in absorption in the sun's atmosphere, but also to absorption in our own. We have been used to think at 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 tie in the blue end than in the red, — another evidence of the great proportionate loss in the

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 it 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 corresponding 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 visible, and both with an instrument that would discriminate 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 invent 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 provided 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 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 lamented friend, Sir William Siemens, (or 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 experimenls were made; among other substaucca, 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 unsuitable, I learned that iron had been rolled of extraordinary 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 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 Rumford. 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 caae, 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 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 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 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 (»r 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 most pass to the scene of their use.

We have been compared to creatures living at the bottom of the sea, who frame their deceptive traditional 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 stilt 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