[b.\rnes] temperature RECORD OF NOCTURNAL RADIATION 1^9 



It was found that by successive reflection of the heat waves on 

 quartz or sylvine surfaces that very long waves could be isolated and 

 studied. 



A very good statement of the limits of our present knowledge in 

 regard to the long heat waves has been given by Professor E. F. jSTichols, 

 of the University of Columbia, in a ipaper entitled, The TJnobtained 

 "Wave Lengths between the Longest Thermal and the Shortest Electric 

 Waves yet ]\Ieasured, which was presented at the International Electrical 

 Congress at St. Louis in 1904. In considering the present boundaries 

 he points out the great advance which has been made in the measure- 

 ment of the long waves by successive reflection from crystalline surfaces, 

 such as quartz. After five reflections on sylvine surfaces, waves of 

 61 f.1 in length have been i.-^olated and measured. In character these 

 long waves resemble electric waves more closely than they do light 

 ^^aves. All metallic surfaces reflect them about equally and almost 

 entirely. The relations between reflecting power and electric con- 

 ductivitv' and between refractive index and dielectric constant hold 

 more rigidly than in light waves. It has been possible to demonstrate 

 with conducting areas of suitable dimensions the same laws of resonance 

 for heat waves which were known previously only for electric waves. 



Professor Nichols further questions how much farther the method 

 of isolation by multiple reflection which has yielded so much can be 

 carried. Substances are known which should have regions of absorption 

 and metallic reflection beyond 60 yu ; but the difficulty of experimenting 

 is very great. Rubens has calculated that from a black body at 2000° C. 

 the intensity of the radiation of waves of the length of 1-5 ja is 800,000 

 times greater than for waves of 60 fx. If the total energy between 

 wave lengths 50 /.i and 60 fx be taken as unity, the total energy between 

 60 fj. and 100 pi will be 0-7, and between 100 )x and 1000 fx only 

 about 0-2. 



It will be seen then how small a proportion of the very long waves 

 exist compared to the shorter waves. Turning to the electric spectrum. 

 Professor Xichols points out that, beginning with the 60 cm. waves of 

 Hertz, Righi, Lebedew and Lampa have successively reduced them. 

 Lampa, using an apparatus differing in no essential respect from the 

 infra-red grating spectrometer, was able to obtain and make measure- 

 ment with waves only 4 mm. long. 



Summing up the complete ether spectrum, Professor Nichols 

 shows, beginning with Schumann's and Lyman's short ultra-violet waves 

 of the order of 'Ifx , that we have about two octaves in the ultra-violet, 

 one in the visible, and six in the infra-red spectrum, making nine in 

 all. The shortest electric waves yet produced begin about six octaves 



Sec. III., 1906. 9. 



