BEYOND THE BED IN THE SPECTRUM — BABCOCK 173 



the latest measurements of this part of the sohir spoctruni fiimi 

 observations made l\v them at Mount Wilson in IU'28. 



The bolometer depends on the chan<;e of electrical resistance shown 

 by a metal whose temperature is varied. A delicate electrical record- 

 ing; device draws a continuous curve whose ordinates indicate the 

 temperature of a hairlike filament of blackened platinum, across 

 which the solar spectrum is made to pass. Figure 3 shows such a 

 curve obtained by Doctor Abbot. The electrical connections are so 

 made that wlien the curve dips down it means that the filament 

 became cooler. Thus these notches mean dark lines or bands inter- 

 ruptinp: the continuity of the spectrum, exactly similar to the well- 

 known Fraunhofer lines in the visible spectrum of the sun. The wave 

 len<j:ths here recorded are from a7X)00 at one end to about A20,000 

 at the other. The thermometric method is capable, as you see, of 

 working over an enormous range of wave lengths, greater in fact 

 than the photographic method. It has another advantage in that 



Fif.tJnK 3. — Boloniclric record of 8olar rner>;y in tlic infra-rod, niadp liy 

 Dr. C. G. Abbot. Beginning nt tJie left wltli very doop red vi.«ihle 

 radiation, the curve extends to tlie right far beyond the limits of 

 visibility and also beyond tho [irpscnt limit of photography 



it gives, more directly than the photographic method, the intensity' 

 of the radiation at each point in the spectrum. On the other hand 

 photography permits the observation of finer details than the ther- 

 mometric method, and leads to higher accuracy. A real advantage 

 is found, therefore, in observing, in so far as possible, the same part 

 of the solar spectrum by both the thermometric and the photo- 

 graphic methods. The greater accuracy of photograj)hy serves to 

 calibrate the wave-length scale of the bolometer, while the bolometric 

 record shows that the spectral lines recorded by the photographs are 

 real and not .spurious effects introduced by the apparatus. 



Nearly 50 years ago Sir William Abney photographed the invisible 

 solar spectrum as far as the point indicated on the curve — a0,867. 

 Attempts to extend his work have failed, although a few years ago 

 Dr. F. S. Brackett, then at the Mount Wilson Observatory, succeeded 

 in reaching a i)oint just short of Abney's limit. We shall now con- 

 sider some recent advances in the pliotographic method which iiave 

 made it possible to use it as far as All,634. 



