330 



Capt. Abney and Col. Festing. 



[June 21, 



only wisli our report of its behaviour might have been more 

 favourable. 



Having confirmed the correctness of Langley's wave-lengths as far 

 as \22,000, we proceeded by means of the thermopile to examine 

 the spectrum obtained when using the crater of the positive pole of 

 the electric light as a source of radiation, and also subsequently the 

 variations caused by placing different thicknesses of water in front of 

 the slit. The thermopile we used was constructed specially for us by 

 Elliott Brothers, some two years ago, when we had made a commence- 

 ment of this work.* It is a linear pile of twenty- six couples to the inch, 

 and one face is covered by a silvered slit with moveable jaws. The 

 couples are mounted in the usual manner, and enclosed in a chamber 

 with transparent ends which can be exhausted, if necessary. Outside 

 this again is a water jacket, through which a constant stream of cold 

 water can be made to circulate. The instrument is extremely delicate, 

 and answered our purpose well. Two reflecting galvanometers were 

 used at different times, one having an internal resistance of about 

 0'5 ohm, and the other of 0*12 ohm. The thermopile was mounted 

 on a stand, moveable horizontally by a screw of 18J turns to the inch. 

 The range of movement was 6 inches. The pile was always moved 

 in one direction during one set of observations. To form the spectrum 

 the following arrangement was adopted. A condenser of white glass 

 threw an image of the crater on the slit of a collimator having a focal 

 length of 20 inches, the lens of the collimator had a diameter of 

 1J inches, and the rays of light were rendered parallel for C, and 

 then fell on a glass prism (sometimes two) placed at approximately 

 the angle of minimum deviation for that ray. A lens of 20 inches focal 

 length attached to a camera formed the spectrum, which could be 

 received either on a sensitive plate or on the face of the pile. The 

 width of the collimator slit was ^ of an inch for the electric lights, 

 and for the sun inch. The width of the slit of the thermopile 

 was in the former case inch, and in the latter In order to 



obtain fiducial points in the spectrum, the arc was focussed on the 

 slit, a sodium spectrum formed, and a photograph taken. The sensi- 

 tive plate was replaced by the thermopile, and the D line brought in 

 the centre of the jaws of the silvered slit, and this point on the 

 screw was used as the zero of the scale. From measurements of the 

 photographs the exact position of all lines and bands could be readily 

 .obtained and referred to the reading taken with the thermopile. A 

 check photograph and zero reading were taken at the end of each 

 series of observations. An assistant kept the image of the crater on 

 the slit (which had a length of '31 inch), one of us attended to the 

 movement of the pile, while the other took the galvanometer readings. 

 The unused face of the pile Avas covered with thick folds of non-con- 

 * " Nature," vol. 25, 1881-82. 



