40 



SIX LECTURES ON LIGHT. 



the electric lamp, L a converging lens, B the 

 bircfracting spar, and P the thermo-electric 

 pile.) 



If time permitted we might finish the series 

 of demonstrations by magnetizing a ray of 

 heat as we magnetized a ray of light. 



We have finally to determine the position 

 and magnitude of the invisible radiation 

 which produces these results. For this pur- 

 pose we employ a particular form of the 

 thermo-electric pile. Its face is a rectangle, 

 which by movable side-pieces can be ren- 

 dered as narrow as desirable. Throwing a 

 concentrated spectrum upon a screen, 

 by means of an endless screw, we move this, 

 rectangular pile through the entire spectrum. 

 Its surface is blackened so that it absorbs all 

 the light incident upon it, converting it into 



a curve which exhibits the distribution of 

 heat in our spectrum. It is represented in 

 the adjacent figure. Beginning at the blue, 

 the curve rises, at first very gradually; then, 

 as it approaches the red more rapidly, the 

 line CD representing the strength of the ex- 

 treme red radiation. Beyond the red it shoots 

 upwards in a steep and massive peak to B, 

 whence it falls, rapidly for a time, and after- 

 wards gradual'y fading from the perception 

 of the pile. This figure is the result of 

 more than twelve careful series of measure- 

 ments, for each of which the curve was con- 

 structed. On superposing all these curves, 

 a satisfactory agreement was found to exist 

 between them. So that it may safely be 

 concluded that the areas of the dark and 

 white spaces respectively represent the rela- 



FIG. 25. 



heat, and thus enabling it to declare its power 

 by the deflection of the magnetic needle. 



When this instrument is brought to the 

 violet end of the spectrum, the heat is found 

 to be almost insensible. As the pile grad- 

 ually moves from the violet towards the red, 

 it encounters a gradually augmenting heat. 

 The red itself possesses the highest heating 

 power of all the colors of the spectrum. 

 Pushing the pile into the dark space beyond the 

 red, the heat rises suddenly in intensity, and.at 

 some distance beyond the red, attains a max- 

 imum. From this point the heat falls some- 

 what more rapidly than it rose, and afterwards 

 gradually fades away. Drawing- an hori- 

 zontal line to represent the length of the 

 spectrum, and erecting along it, at various 

 points, perpendiculars proportional in length 

 to the heat existing at those points, we obtain 



I tive energies of the visible and invisible 

 I radiation The one is 7.7 times the other. 



But in verification, as already stated, con- 

 sists the strength of science. Determining 

 in the first place the total emission from the 

 electric lamp ; then by means of the iodine 

 filter determining the ultra-red emission ; the 

 difference between both gives the luminous 

 emission. In this way, it was found that the 

 energy of the invisible emission is eight times 

 that of the visible. No two methods could 

 be more opposed to each other, and hardly 

 j any two results could better harmonize. I 

 I think, therefore, you may rely upon the ac- 

 j curacy of the distribution of heat here as- 

 signed to the prismatic spectrum of the elec- 

 i trie light. There is nothing vague in the 

 j mode of investigation, nor doubtful in its 

 conclusions. 



