TRANSACTIONS OP THE SECTIONS. 



43 



baps we may obtain assistance in sucb cases from disposing tbe apparatus so as to 

 present to tbe eye spectra of tbe First Order and of tbe Second Order in rapid suc- 

 cession (see Plucker, Pbil. Trans, for I860, p. 10. § 24). 



But tbere seems to be at least one case of a more rapid periodic time, viz. tbat of 

 certain lines of bydrogen. Tbe ordinary spectrum of tbe Second Order of tbis gas 

 consists of tbe four well-known lines C, F, one near G, and h. Tbree of tbese 

 lines, viz. C, F, and h, are to be referred to a single motion in tbe molecules of 

 tbe gas. In fact tbey are tbe 20tb, 27tb, and 32nd barmonics of a wave-lengtb 



{in vacuo) of 1-31217714 fiftb-metre (tbe Vtb-metre being — ^ of a metre, wbicb 



is a little more tbau tbe diameter of a disk of liuman blood). Tbis, taking tbe ve- 

 locity of ligbt to be 298 millions of metres, gives 4-4 XlVtb-seconds as tbe periodic 

 time of tbat motion in tbe molecules of bydi'ogen in wbicb tbese tbree lines have 

 tbeir origin. 



Tbis determination may be accepted as very close to tbe truth. Tbe most un- 

 certain part of it is the velocity which has been assigned to light. The rest of the 



computation depends on Angstrom's marvellously accurate measures of the wave- 

 lengths in air of standard pressm-e and 14° C. temperature, reduced to wave-lengths 

 in vacuo by Ketteler's observations on the dispersion of air ; and the following Table 

 will show bow verv close the calculated values are to those which were observed. 



The difference in no case amounts to an eleventh-metre, which is the limit -within 

 which Angstrom thinks that bis measures may be depended on. 



Possibly some of the other barmonics, such as tbe 19th, 21st, 22ud, &c., which 

 are not visible in the ordinary spectrum of hydrogen, may be found among tbe lines 

 of that other spectrum of many lines which Plucker has recorded (Phil. Trans, for 

 I860, p. 22. § 60). 



A more detailed examination of the foregoing theory raises the hope that it will 

 throw light from various directions upon molecular motions. 



Experiments on Colour. 

 By the Hon. J. "W. Sxruxt, Fellow of Triniti/ Goller/e, Cambriclje. 

 The author gave an account of some observations -with tbe colour-disks, and on 

 the disturbance produced by looking through coloured solutions at tbe match first 

 adjusted by the naked eye. A difference in the coloui--equations was also noticed ac- 

 cording to whether the light came from the blue sky or from clouds. Some remarks 

 followed on tbe nature of the compound yellow ; and a method of obtaining it was 

 described more convenient in many cases than the use of the spectral rays isolated 

 by tbe prism. Mixtures of either an alkaline infusion of litmus or a solution of 

 sesquicbloride of chromium -with the chromate of potash isolate tbe green and red 

 portions of tbe spectrum, cutting out the yellow and orange which lie between 

 them. In suitable proportions, either of them give a very full compound yellow, 

 which is yet almost entirely free from any yellow-looking elementary light. 



