Manchester Memoirs, Vol. Iviii. (19 14), No. 15. 3 



consists of many strong lines on an almost continuous 

 background. 



The Containing Vessel. Hartley used small quartz- 

 sided cells which held a definite thickness of solution. 

 Baly has, however, effected a great improvement. The 

 Baly tube consists of two concentric glass tubes, one end 

 of each of which is closed with a quartz plate. A piece 

 of rubber tubing serves to make a tight joint, and to 

 allow the smaller tube to move freely inside the larger. 

 The latter is provided with a bulb which acts both as an 

 entrance and as a reservoir to the apparatus. The side 

 of the larger tube has engraved on it either thicknesses in 

 millimeters or the logarithms of corresponding thick- 

 nesses. 



The Spectrograph. Any spectroscope will do if fitted 

 with an optical train transparent to ultraviolet light and 

 with a camera attachment, but it is desirable that the dis- 

 persion should give a spectrum about seven inches long 

 from wave length A = 8,000 to 2,000,* all of which should 

 be in focus at the one time. 



Solvents. The ideal solvents are those which are per- 

 fectly transparent between the above-mentioned limits. 

 It is doubtful, however, if any do fulfil that condition, 

 although some such as water, alcohol, and chloroform 

 show, in the pure condition, very little absorption. Still, 

 alcohol usually begins to show absorption at 3,200, and 

 bad specimens may transmit a weakened spectrum over 

 the whole of the ultraviolet. 



Plates. Most plates are sufficiently sensitive down to 

 X = 2,500, beyond which region the photographic image 

 falls off in intensity, partly through the weakness of the 

 rays but probably to a greater extent through absorption 

 by the materials of the plate. In the examination of this 



* All wave lenglhs are given in Angstrom units 



