6 S o THE POPULAR SCIENCE MONTHLY. 



Bon-transparent substances like mother-of-pearl, where the iridescent 

 hues are due to microscopically fine lines. Art has lately surpassed 

 Nature in these wonderful "gratings," consisting of pieces of polished 

 metal, in which we see at first nothing to account for the splendid 

 play of color 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 fifty to one hundred in the thick- 

 ness 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. Tou will see, however, 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 spectrum 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 can not 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 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 con- 

 nection 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 clue 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 another 

 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 atmos- 

 phere, 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 Fraun- 

 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, 



