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SCIENCE. 



[N. S. Vol. II. No. 33. 



mains stationary. If one of the strips be 

 now warmed, ever so slightly, its resistance 

 will be increased, the balance is destroyed, 

 a current of electricity traverses the gal- 

 vanometer and the beam of light is deflected 

 from its normal position (zero). The de- 



FlG. 2. Diagram of Wheatestone bridge showing 

 how the two resistances c and d are replaced by the 

 ' side arm ' n and the ' middle arm ' m of the bo- 

 lometer. 



flection is a measure of the heating of the 

 strip, and the position of the spot of light 

 upon a scale always tells the thermal 

 condition of the strip. Such a pair of 

 strips, fitted with many devices and spe- 

 cial adaptations, constitutes a ' bolome- 

 ter.' One strip, called the ' central arm,' 

 is exposed to the radiations to be measured, 

 while the other, ' side arm,' is carefully 

 shielded therefi'om. The exposed strip of 

 a bolometer for specti'um work is about 8 

 mm. long, ^V mm. wide, and -^l^ mm. thick 

 {h X 6^5 X ^ijVo inch ) appearing like a 

 fine hair. Of course the galvanometer is 

 the most delicate, and aU precautions are 

 taken. Such a system will record a rise in 

 temperature of one of its strips of less than 

 TSijicTTTT of a degree centigrade (^jytyViTij Fh.) 

 Professor Langley's pioneer work tuto 

 the infra-red regions of the spectrum was 

 all by visual observations with the bolom- 



eter. Let us suppose that some invisible 

 Fraunhofer line is to be located. A large 

 clockwork arranged to rotate a mirror is so 

 adjusted that it reflects a beam of sun- 

 light upon the slit of the spectrometer, 

 which then gives a distinct spectrum, with 

 its lines visible and invisible. The bolom- 

 eter, mounted upon the arm of the spec- 

 trometer, is so set that its central arm 

 coincides with some visible Fraunhofer 

 line, and the circle of the spectrometer is 

 read. This is the starting point. The arm 

 is then turned until the bolometer stands 

 where a line is sought. Again the circle 

 is read. The slit of the spectrometer is 

 then closed and the position of the spot of 

 light at the galvanometer is noted. The 

 slit is opened and the galvanometer read, 

 then the slit is closed and the galvanometer 

 read. The average of the first and third 

 galvanometer readings subtracted from the 

 second gives the deflection due to the radi- 

 ant energy falling upon the bolometer at 

 that particular point in the invisible infra- 

 red spectrum of the sun. The arm is now 

 moved forward a little, bringing the bo- 

 lometer into a new part of the spectrum. 

 Again a series of deflections are read and 

 the energy measured. Thus hundreds and 

 thousands of points in the spectrum are 

 determined, and these, when plotted, show 

 the deep valleys where the energy runs low 

 and the hills where it is abundant. In this 

 way Professor Langley and his assistants, 

 with consummate patience and persever- 

 ance, felt over the long stretches of the 

 infra-red, mapping a beautiful ' energy 

 curve,' with its many little notches and its 

 four or five huge valleys separated by high 

 peaks. 



Some time after Prof. Langley's advent 

 in Washington he organized an astrophys- 

 ical observatory and in it has prosecuted his 

 investigations by a new method and with 

 renewed enthusiasm. The essential prin- 

 ciples of the operation remain the same. 



