TRANSACTIONS OF THE SECTIONS. 



41 



In each observation three colours of the spectrum are mixed and so adjusted that 

 their mixture is so exactly equivalent to the white light beside it, that the line 

 which divides the two can no longer be seen. 



It is found that in certain cases, when this adjustment is made so as to satisfy 

 one person, a second will iind the mixed colour of a green hue, while to a third it 

 will appear of a reddish colour, compared with the white beam. 



But, besides tliis, it is found that the mixed colour may be so adjusted that, if 

 we look directly at it, it appears red, while if we direct the eye away from it, and 

 cast a sidelong glance at it, we see it green. The cause of this is the yellow spot, 

 which acts somewhat as a piece of yellow glass would do, absorbing certain kinds 

 of light more than others ; and the difference between difl'erent persons arises from 

 different intensities of the absorbing spot. It is foimd in persons of every nation, 

 but generally stronger in those of dark complexion. The degi-ee of intensity does 

 not seem to depend so much on the colour of the hair or the iris of the individual, 

 as to run through families independent of outward complexion. 



The same difference is found between different colour-blind persons ; so that in 

 the comparison of their vision with that of the normal eye, persons should be 

 selected for comparison who have the yellow spot of nearly the same intensity. 



In my own eye the part of the spectrum from A to E is seen decidedly 

 better by the central part of the retina than by the surrounding parts. Near F 

 this is reversed, and the central part gives a sensation of about half the intensity 

 of the rest. Beyond G the central part is again the most sensitive, and it is decidedly 

 so near H. 



Before I conclude I wish to direct the attention of those who wish to study 

 colour to the exceedingly simple and beautiful series of experiments described by 

 Mr. W. Benson in his works on colour. By looking through a prism at the 

 black and white diagrams in his book, any one can see more of the true relations of 

 colour than can be got from the most elaborately coloured theoretical arrangements 

 of tints. 



On the Cause of the Interrupted Spectra of Gases. 

 By G. JoHNSTOKE STo^'Er, M.A., JF.B.S. 



In 1867 ]\Ir. Stoney had instituted a comparison between the internal motions 

 of gases and the phenomena of light *, from which it appeared that the lines in 

 the spectra of gases must be attributed to periodic motions within the mole- 

 cules, and not to the little irregidar journeys which the molecules make amongst 

 one another. 



In the present communication he endeavoured to advance another step. Each 

 such periodic motion in the gas will throw off waves in the aether. These 

 waves will in general not be pendulous ; in fact the circumstances would need 

 to be altogether exceptional which would restrict them to so simple a form. 

 But whatever the character of each wave, provided only that all the waves of 

 the imdulation are alike, it follows from Fourier's well-known theorem that the 

 Undulation may be adequately represented by the simultaneous advance of a 

 number of superposed pendulous imdulations — one consisting of waves of the 



■* See a paper in the Philosophical Magazine for August 1868, p. 132. Eeaders of that 

 paper are requested to correct " 16'' or upwards of 250 " (on p. 134), into " V 16 "• 



