42 2 



NATURE 



[January 26, 191 1 



shorter wave-length by the same condition. In this way 

 the whole spectrum is divided into a number of contiguous 

 divisions, or patches, which Dr. Green terms mono- 

 chromatic. 



" Tested with this instrument a normal individual will, 

 as a rule, name six distinct colours (viz. red, orange, 

 yellow, green, blue, violet), and will mark out by means 

 of the shutters about i8 monochromatic patches. 

 Occasionally we come across individuals with a greater 

 power of differentiating hues, to whom, as to Newton, 

 there is a distinct colour between the blue and violet, 

 which Newton called indigo. .Such individuals will mark 

 out a greater number of monochromatic patches, from 

 22 up to 29. The limited number of monochromatic 

 patches which can be marked out in this way is 

 at first surprising when we consider how insensibly 

 one part of the spectrum seems to shade into the ne.x.t 

 when the whole of the spectrum is looked at. The number 

 and position of the patches present, however, great 

 uniformity from one case to another." 



Being curious to know into what class my own vision 

 would fall on this system, I was glad to be tested by 

 Dr. Green last July. The number of patches proved to be 

 17, a little short of what Dr. Green lays down in the 

 passage aBove quoted as normal. The limits of the actual 

 patches were as follows : — 



780—635^—624—612—603—595—586—576—560—541— 

 521—509-500—489^-477-462-443—426. 



Thus in the region of the D lines a patch including 

 wave-lengths between 595 and 586 did not manifest a 

 difference of colour. The interval between the D lines on 

 the above scale being o-6o, it appears that my " mono- 

 chromatic patch " was 15 times this interval. 



While it is undoubtedly true that in this way of work- 

 ing no colour-difference was perceptible as the eye travelled 

 backwards and forwards over the patch, my experience 

 with colour discs and other colour-mixing arrangements 

 made me feel certain that under more favourable condi- 

 tions I could discriminate much smaller differences of 

 wave-length. Special ex{>eriments have since proved that 

 I can, in fact, discriminate by colour between points in 

 the spectrum so close together as the two D lines. 



In order to compare two colours with advantage it is 

 necessary that each should extend with uniformity over a 

 considerable angular area, and that the two areas should 

 be in close juxtaposition. The requirements of the case 

 are sufficiently met by a colour-box (after Maxwell) such 

 as I described nearly thirty years ago.' In this form of 

 apparatus a second slit, placed at the focus, allows a 

 narrow width of the spectrum to pass ; but instead of 

 regarding the transmitted portion with an eye-piece, the 

 eye is brought close to the slit and focussed upon the 

 prism, which thus appears uniformly lighted with such 

 rays as the second slit allows to pass. The light thus 

 presented is, of course, not absolutely homogeneous ; it 

 Includes a mixture of neighbouring spectrum rays, the 

 degree of purity augmenting as the slits are narrowed. 

 With the aid of a refracting prism of small angle (set 

 perpendicularly to the dispersing prisms) the field of view 

 IS divided into two parts, which correspond to any desired 

 colours according to the ' situation of the two primary 

 slits. For the present purpose these primary slits lie 

 •nearly in one straight line, inasmuch as the two spectrum 

 colours to be compared are close together. 



In making the observations on sensitiveness, one 

 primary slit, as well as the eye-slit, remains fixed, the 

 position being chosen so as to provide yellow light from 

 the neighbourhood of D. The second slit can be moved 

 as a whole while retaining its width. 



The procedure is quite simple. If the colours seen are 

 strongly contrasted, the movable slit is displaced until the 

 difference is moderate. Marks may then be given : O, 

 denoting that the difference is uncertain; R,, that it is 

 just distinct in the direction of making the second patch 

 the redder; G,, that it is just distinct in the opposite 

 direction. Similarly, R,, G,, denote differences in the 

 two directions which are more than distinct, and so on. 

 After each observation worth recording, the position of the 

 movable slit is measured. 



1 Nature, 1881, vol. xxv., pp. 64-66 ; "Scientific Papers," vol. i., p. 543 

 See also Nature, August 18, 1910. 



In this manner, as the result of sets of observations 

 made on several days, it was found that a movement of 

 the second slit through 01 5 mm. was sufficient to carry 

 the variable colour from being distinctly redder than the 

 standard to distinctly greener. We may conclude that the 

 eye is capable of appreciating without fail a difference of 

 situation represented by 0-07 mm. 



It remains to interpret the result in terms of wave- 

 lengths. By allowing light to enter at the eye-slit, ur 

 rather at a narrower slit superposed upon it, a spectrum 

 is formed at the other end the scale of which has to be 

 determined. It appeared that the distance from D to E 

 was 7 mm. The difference of wave-length between these 

 lines is 62-3. The perceptible difference is i/ioo of this, 

 corresponding nearly enough to the difference between the 

 D lines. I think I am safe in saying that I could dis- 

 tinguish the colours of the two D lines if favourably 

 presented to the eye. 



This degree of sensitiveness, though not higher than I 

 had expected, is a little difficult to reconcile with the 

 monochromatic appearance of a portion of the spectrum 

 fifteen times wider. I suppose that the gradual character 

 of the transition is an obstacle to the recognition of differ- 

 ences. The question of angular magnitude may also enter. 

 No doubt a very small apparent magnitude would be un- 

 favourable. It is possible that in Dr. Green's apparatus 

 an eye-piece of higher power, with a corresponding 

 augmentation in the intrinsic brilliancy of the source of 

 light, would allow of an increase in the number of dis- 

 tinguishable patches. The experiment would be worth a 

 trial. 



It will be seen that the existence of " monochromatic 

 patches " in the spectrum is far from meaning that thp 

 eye is incapable of making chromatic distinctions within 

 their range. I do not infer from this that the results of 

 the method are without significance. Undoubtedly it i^ 

 possible by means of it to classify colour-vision, and such 

 a classification cannot be without interest, even if we fail 

 as yet to understand exactly what it means. 



THE PROGRESSIVE DISCLOSURE OF THE 

 ENTIRE ATMOSPHERE OF THE SUN."^ 



T E soleil auquel est consacr6e cette conference est un 

 magnifique sujet d'^tudes. Tous les hommes sentent 

 plus ou moins claireinent que les destinies terrestres sont 

 liees 6troitement k celles du soleil, et qu'il est ndcessaire de 

 reconnaitre sa nature intime, son rayonnement total, ses 

 variations, en un mot son action precise et complete sur 

 nolle globe. Notre d^pendance vis-i-vis du soleil est 

 absolue, et r^cemment, elle a 6t6 r^sum^e d'une mani^re 

 simple par un homme politique frangais, maintenant 

 ministre dos finances, auquel je demandais un credit special 

 pour robservatoire de Meudon que je dirige, et pour les 

 recherches solaires. II refusait d'abord, en all^guant 

 I'acroissement continu des d^penses publiques. Puis, comme 

 j'insistais, il s'^cria : " V'ous avez raison, le soleil est notrc 

 tnaitre a tous ; il est impossible que nous ne fassibns pas 

 quelque chose." C'cst ainsi que I'observatoire de Meudon 

 a pu joindre k son budget ordinaire une somme suppl^- 

 mentaire, certes peu eiev(5e, mais qui est arriv^e au moment 

 opportun, et nous a beaucoup aid^s dans les recherches que 

 je vous pr^sente aujourd'hui. 



L 'etude moderne du soleil exige en effet des installations 

 couteuses, des appareils compliqu^s et un personnel special 

 apte aussi bien aux observations physiques qu'aux observa- 

 tions astronomiques. Or le soleil luit pour tout le monde. 

 et miirit toutes les moissons ; et, k priori, il semble nature! 

 que tous les hommes de la plan^te apportent leur concouf^ 

 aux recherches solaires. Partant de cette id^e, j'ai propose, 

 il y a quelques ann^es, k la Soci6te astronomique de Franc 

 une taxe sp^ciale ct g^n^rale pour le soleil — et d'ailleur= 

 tr^s minime. Si chaque fran<jais, ai-je remarqu^, donnait 

 par an un sou, un simple sou pour le soleil, la somme 

 totale serait encore eiev^e ; elle permettrait d 'assurer 

 I'enregistrement continu du soleil et de ses variations, non 

 encore realise, et done une connaissance plus approfondif 

 de I'astre. Mais les taxes nouvelles sont toujours_ plu- 

 nombreuses, et celle-li, bien que tr6s faible et tr^s legitime, 



1 Discourse deliv-red at the Royal Institution of Great Britain on Fri 

 June 10, 1910, by Dr. H. Deslandres, Membre de I'lnstitut, 



NO. 2152, VOL. 85] 



