1882.] Captain W> de W. Abney on Spectrum Analysis^ dtc. 57 



WEEKLY EVENING MEETING. 



Friday, Marcli 17, 1882. 



George Busk, Esq. F.R.S. Treasurer and Vice-President, 



in the Chair. 



Captain W. de W. Abney, E.E. F.R.S. 

 Spectrum Analysis in tJie infra red of the Spectrum. 



At the Royal Institution it would be almost an impertinence on my 

 part were I to attempt to prove the existence of dark rays which lie 

 below the red of the sjDCctrum : Professor Tyndall has made you all 

 so thoroughly acquainted with these dark rays that I may assume not 

 only your acquaintance with them, but also your interest in them. 

 The most accessible means hitherto of exj)loring this region of the 

 spectrum has been by a study of its heating effect, as shown by the 

 thermopile ; and more particularly when an integration of the effect 

 produced by its component rays is required. Beautifully delicate is 

 the thermopile, but it must not be forgotten that it depends for its 

 delicacy largely on the area of its face. A little consideration will 

 show that if you wish to examine the spectrum by its means, you 

 must be prepared for a drawback. 



In the thermopile I have here, the elements composing it are 

 arranged in a line, and I can allow any fineness of beam to penetrate 

 to its surface by means of this adjustable slit. Unfortunately, how- 

 ever, the narrower the slit the smaller is the heating effect on tho 

 thermopile ; so that, if I wish to measure the heating effect of a very 

 narrow beam of radiation which may find its way between the jaws of 

 the slit, the heating effect will be so small that it may escape detection. 

 In the visible solar spectrum, as you are all aware, there are breaks 

 of continuity presenting to the eye the appearance of fine dark lines, 

 and showing a diminished radiation. Now, suppose for an instant 

 that the part of the spectrum which is visible to us was dark, not 

 exciting vision, it is manifest, even were the energy of this part of 

 the spectrum very much greater than it is, that to ascertain the 

 existence of these fine lines by means of the thermoj)ile would be 

 next to impossible; since the slit would have to be closed to an 

 excessive degree of fineness, and when so closed the thermopile 

 would be insensible to radiations when such existed. Now this is 

 precisely the case with which we have to deal in the spectrum below 

 the red. We know of its existence ; but with any source of radia- 

 tion, such as the sun for instance, the thermopile would stand but a 

 small chance of finding any narrow breaks in its continuity. I need 



