NATURE 



[January 9, 1896 



the three fundamental sensations, is made only for the sake of 

 greater convenience in discussion. 



Indeed there is still much for us to learn regarding the nature 

 of colour sensation. Among the yet unexplained phenomena 

 are those of simultaneous colour contrast. The fact that a small 

 brightly-coloured area on a grey background appears surrounded 

 by its complementary tint is familiar enough. For its explan- 

 ation it has been common to assume that there is unconscious 

 motion of the observer's eyes, incipient retinal fatigue, an error 

 of judgment, or fluctuation of judgment. This has been tested 

 by A. M. Mayer {American J ounial of Sciettce, July 1893), ^^o 

 ingeniously devised methods for showing these contrast 

 phenomena on surfaces large enough to match the colours with 

 those of rotating colour discs, and thus to arrive at quantitative 

 statements of their hues. When viewed through a small open- 

 ing in a revolving disc the subjective contrast colour was unmis- 

 takably perceptible when the duration of passage of the open- 

 ing was less than -^-^-^-^ of a second. The same effect was 

 obtained in a dark room with instantaneous illumination of the 

 coloured surface by the strong spark of an electric influence 

 machine. The duration of illumination is thus almost in- 

 finitestimal, certainly not more than xinro^ti-oTT of a second. 

 The hypothesis of fluctuation of judgment is thus shown to be 

 wholly untenable. I have performed most of these experi- 

 ments, either with Prof Mayer or separately, and my testimony 

 can therefore be united with his. The case is quite analogous to 

 that of the perception of binocular relief, which was once ex- 

 plained as the product of a judgment, but was found to be always 

 possible with instantaneous illumination. Prof. Mayer has 

 devised a disc photometer based on colour contrast, with which 

 the error of a single reading was found much less than with the 

 Bunsen photometer. 



The rotating colour disc has been applied by O. N. Rood 

 {American Journal of Science, September 1893) to the deter- 

 mination of luminosity independently of colour, by taking 

 advantage of the flickering appearance on a rotating disc upon 

 which two parts have different reflecting powers. An extreme 

 case of this is that of a white sector upon a black disc. At a 

 certain critical sjieed the retinal shock due to momentary im- 

 pression by white light becomes analysed into the subjective 

 impression of spectral colours, the duration of the retinal 

 sensation varying with the wave-length of the incident light. 

 The law of this variation has been studied by Plateau ( ' ' Dis- 

 sertation sur quelques proprietes des impressions produits par la 

 lumiere sur I'organe de la vue," Liege, 1829), Nichols (^wer/Va« 

 Journal of Science, October 1884), and more recently with much 

 precision by Ferry {ibid., September 1892), who showed that 

 retinal ]>ersistence varies inversely as the logarithm of the 

 luminosity. Yox a given source of light separated into its 

 spectral components, the yellow is the brightest. For this 

 hue accordingly the retinal impression is shortest, and for violet 

 it is longest. 



Under appropriate conditions the after-effect on the retina has 

 a certain pulsatory character, as first noted by C. A. Young 

 {Philosophical Magazine, vol. xliii. p. 343, 1872) in 1872, and 

 carefully studied within the last few years by Charpentier (" 0%- 

 dilations retiniennes," C<7w/>^£j r,?«^i«, vol. cxiii. p. 147, 1891) 

 in France, and Shelford Bidwell (" On the Recurrent Images 

 following Visual Impressions," Proc. Royal Society, March 27, 

 1894) in P^ngland. A disc with properly arranged black and 

 white sectors, if brightly illuminated and looked at while revolv- 

 ing at a moderate rate, becomes apparently coloured, just as a 

 momentary glance at the sun causes the perception of a succes- 

 sion of subjective spectral hues which may last a number of 

 seconds. The phenomenon in relation to the disc was known 

 as early as 1838 (Fechner, Poggendorff s Annalen, 1838), and 

 ex]3lained by Rood {American Journal of Science, September 

 1 860) in 1 860. The re-discovery of what has been long forgotten 

 arouses all the interest of novelty. The " artificial spectrum 

 top," devised by Benham (Nature, November 29, 1894, p. 113) 

 last autumn, excited interest on two continents, and was promptly 

 copy righted by a prominent firm of opticians {ibid., March 

 14, 1895, P- 463) in England. It would perhaps be equally 

 enterprising to copyright the solar spectrum. 



The limits of a single address forbid my touching upon the 

 large and practically important subject of colour blindness. 

 Indeed, in both physical and physiological optics much has been 

 omitted that is abundantly worthy of attention. In behalf of 

 my hearers it may be wise to take heed, once more, of the fate 

 •of Tarpeia, who was overwhelmed with the abundance of her 

 reward. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 



The Technical Education Board of the London County 

 Council has just awarded 278 minor scholarships, viz. 178 to 

 boys and 100 to girls in Elementary Schools ; 334 scholarships 

 of the same class were awarded last spring, upon the results ol 

 examination, so that altogether the number awarded by the Board 

 in 1895 was 612. 



At a meeting of the Fellows of the Royal College of 

 Surgeons, held on Thursday last in the thsatre of the college, a 

 resolution was carried, " that, in the opinion of the Fellows of 

 this college, women should be admitted to the diplomas of the 

 college," forty-seven Fellows voting for the resolution and only 

 ten against. The Fellows alone form the electorate who vote 

 for election to the council, and the effect of this resolution will 

 probably cause the council (who are understood not to be 

 unwilling) to open the examinations to women candidates. At 

 a mixed meeting of Fellows and members, called by the President 

 last November to consider an application from the Dean of the 

 London Medical School for Women for this privilege, Mr. 

 Clement Lucas's proposal to the same effect was negatived by 

 the narrow majority of ten in a house of over a hundred. 



In connection with the new Technical Institute recently opened', 

 at Wandsworth, the London Technical Education Gazette re- 

 calls the interesting fact that the first technical school in this 

 country was opened in Wandsworth. The third annual report 

 issued by the Science and Art Department, in 1856, gives an 

 interesting account of this first technical school, which was called 

 the Wandsworth Trade School. The curriculum included partly 

 subjects of general instruction and partly courses of trade in- 

 struction classified under three heads, according as they had 

 relation to (i) the building trades, (2) the mechanical and en- 

 gineering trades, and (3) the chemical and manufacturing trades. 

 The new Technical Institute will, it is hoped, revive the traditions 

 established by the pioneer school of 1856. In addition to an. 

 equipment grant of ;i^5oo, the Technical Education Board has 

 agreed to contribute ;i^iooo to the maintenance of the institute 

 for the current year, apart from any grants which it may make 

 for the maintenance of the technical day school. 



SCIENTIFIC SERIALS. 



Bulletin of the American Mathematical Society, vol. ii. No. 2.. 

 November 1895. — Concerning Jordan's linear groups, is a paper 

 by Prof E. H. Moore, which was read before the Society in 

 August last. It is a continuation of a paper read in November 1894, 

 entitled "The group of holoedric transformation into itself of a 

 given group " and is an exhaustive one supplemented by numerous 

 bibliographical details.— Prof. A. S. Hathaway presented, at 

 the same meeting in August, an elementary proof of the 

 quaternion associative principle. Hamilton in his " Elements" 

 writes : " The associative principle of multiplication may also be 

 proved without the distributive principle, by certain considera- 

 tions oi rotations of a system, on which we cannot enter here." 

 This noue states that it is easy to see that such a proof is 

 possible; but the details of it could not have presented them- 

 selves to Hamilton in an elementary form, or he would have 

 seen that it was just the demonstration for which he was looking, 

 simple in character, and direct in its application. We are not 

 sure that we have not seen a proof sonfiewhat similar to the 

 Professor's, but we cannot recall it to our recollection. The 

 proof given is a simple one. — The next article is a paper read at 

 the October meeting of the Society, entitled " Moral Values," 

 by Mr. R. Henderson. The author reminds us that the question 

 of moral values in connection with the theory of probability has 

 given rise to great diversity of opinion among mathematicians, 

 and that Bertrand, in his classical work, dismisses it with con- 

 tempt. More than the usual space is devoted to the notes and 

 new publications. 



American Meteorological Journal, December 1895. — Psychro- 

 meter studies, by Dr. Nils Ekholm. This article chiefly refers 

 to the peculiar action of the wet-bulb thermometer near the 

 freezing point of water. The author's observations and other 

 investigations show that in an air saturated with water-vapour, 

 the ice-covered bulb reads higher than the water-covered one, 

 which, under those conditions, reads exactly as the dry bulb. 

 These results are explained by Prof W. Ramsay's experiments, 

 which prove that there is a difference in the tension of water- 



NO. 1367, VOL. 53] 



