272 Professor S. P. Lanr/Iey | April 17, 



spleudid play of colour apparently pouring out from them like light 

 from an opal, but which, on examination with a powerful microscope, 

 show lines so narrow that there are from 50 to 100 in the thickness 

 of a fine human hair, and all spaced with wonderful precision. 



This grating is equal in defining power to many such prisms as 

 we have just been looking at, but its light does not show well upon 

 the screen. You will see, liowever, that its spectrum differs from 

 that of the prism, in that in this case the red end is expanded, as 

 compared with the violet, and the invisible ultra-red is expanded still 

 more, so that this will be the best means for us to use in exploring 

 that " dark continent " of invisible heat found not only in the spec- 

 trum of the sun, but of the electric light, and of all incandescent 

 bodies, and of whose existence we already know from Herschel and 

 Tyndall. 



Now we cannot reproduce the actual solar spectrum on the screen 

 without the sun itself, but here are photographs of it, which show 

 parts of the losses the different colours have suffered on their way to 

 us. We have before us the well-known Frauenhofer lines, due, you 

 remember, not only to absorption in the sun's atmosj)here, but also to 

 absorption in our own. We have been used to think of tliem in 

 connection with their cause, one being due to the absorption of iron- 

 vapour in the sun, another to that of water- vapour 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 ligJit, and that collectively they tell much of the story of 

 wliat 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 — another evidence of the great proportionate loss in the blue. 



If WG 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 atmo- 

 sphere, 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 absorj)ti(m-spaces in it corresponding to the Frauen- 

 hofer 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 dis- 

 criminate the energy in these very narrow spaces, like an eye to sec 

 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 hero 



