Nov. 27, 1879] 



NATURE 



79 



of colour. The reproach laid against the true colour- 

 theory of Newton that it was less fruitful for artistic ends 

 than the false theory of Goethe, is impossible in the face 

 of such modern works as those of Chevreul, Field, Helm- 

 holtz, Briicke, and von Bezold. And now Prof. Rood's 

 new work will be welcomed as an addition to the literature 

 of the subject. 



The first two chapters are devoted to the general laws 

 of light, and of its dispersion by refraction and by diffrac- 

 tion. Then comes a chapter on the three "constants" 

 of colour, purity, luminosity, and hue, the term luminosity 

 being employed, not as artists sometimes employ it to 

 describe a particular " effect " of light and shade in a 

 picture, but as the equivalent of the measurable intensity 

 or brightness of the light. The author avoids the term 

 " intensity " in this sense, that it may not be confounded 

 with the term "saturation," a quality of colour which 

 depends upon both purity and luminosity, and which is 

 also sometimes erroneously spoken of as the " intensity : ' 

 of a colour. The four following sections deal with the 

 production of colour by interference and polarisation, by 

 turbid media, by fluorescence and phosphorescence, and 

 by absorption. The last of these chapters is very care- 

 fully written, and contains spectroscopic diagrams of a 

 number of absorbing media. Their bearing upon the all- 

 important question of the tint transmitted by two coloured 

 media jointly is clearly explained. The remaining 

 chapters are devoted to Young's Theory, Mixture of 

 Colours, Complementary Colours, Colour Systems, &c. 

 A concluding chapter deals with Painting and Decora- 

 tion. 



Following von Bezold, Prof. Rood rejects the term 

 '• indigo " introduced by Newton into the classification of 

 the spectrum colours, and describes the colours between 

 green and violet as blue-green, cyan-blue, blue, and violet- 

 blue. The spectrum line F stands between " cyan-blue " 

 and "blue," while "violet-blue" begins about half-way 

 between F and G, and ends a little beyond the latter line. 

 This classification differs slightly from that of Listing. 



A detailed account is given of Maxwell's Theory of 

 Colours, of the experiments by which he arrived at his 

 results, and of the colour-chart devised by him. It is 

 unfortunate, however, that the author has divided his 

 excellent remarks on this head, giving part in an appendix 

 to Chapter VIII., part in another appendix to Chapter 

 XIV., and the elementary explanation of the method of 

 balancing the colours upon p. 219 of the text. Apart from 

 this awkward arrangement the matter is admirably put ; 

 and is the best exposition of Maxwell's theory in the 

 language. Indeed it is singular that most English text- 

 books ignore Maxwell's work in this department. In the 

 English edition of Deschanel's " Natural Philosophy,'' 

 which is almost the only one which touches the matter at 

 all, the brief paragraph in which the theory is dealt with 

 lacks the perspicacity that mostly distinguishes that well- 

 known work. 



There are one or two sentences in the work which 

 cannot command our assent ; and should be revised when 

 another edition is called for. Thus, on p. 86, we are told 

 that Becquerel and other earlier experimenters succeeded 

 in obtaining fleeting photographs of the colours of the 

 spectrum, but that "the colours thus obtained are pro- 

 duced merely by the interference of light." And again, 



" In blue eyes there is no real blue colouring matter at 

 all" (p. 58). On p. 94 the author claims as his own an 

 experiment described originally in this country by T. 

 Rose, the inventor of the kalotrope. A reference is given 

 on p. 82 to the darkening of tint of water when heated, 

 due to increased absorption : but the author makes no 

 reference whatever to the important observations of 

 Gladstone, Hartley, and Ackroyd on the similar changes 

 which take place in almost all coloured bodies when 

 heated ; nor to the significant observation of the last- 

 named experimenter, that with increasing temperature 

 the absorption appears to increase most in the blue end 

 of the spectrum in the case of those solid bodies of fixed 

 composition which expand with a rise of temperature, 

 while it increases most at the red end for those few 

 bodies such as iodide of silver which contract with a rise 

 of temperature. Hering's theory of colours deserves a 

 more extended notice than the very short note given in 

 the final appendix. A brief account is given on p. 83 of 

 a simple means devised by Simmler for observing the red 

 rays which are abundantly reflected by green leaves : a 

 thick plate of blue cobalt glass in conjunction with a plate 

 of yellow glass serving to cut off all rays except the red 

 and the blue-green. The writer of this notice independ- 

 ently described some few years ago a similar device, in 

 which by taking a solution of permanganate of potash in 

 a glass tank of a convenient size, the blue, green, and 

 yellow rays were similarly absorbed, allowing only red 

 and violet bands to pass, thus constituting, like Simmler's 

 double plate, an erythroscope. 



The portions of Prof. Rood's book which bear upon 

 artists' work are numerous, and his observations are of 

 importance. There is, for example, a careful discussion 

 of the change of visible tint suffered by coloured surfaces 

 under diminished illumination ; and a parallel discussion 

 of the results obtained by mixing pigments with a propor- 

 tion of black. A list is given of those pigments which 

 are liable to change or fade by exposure. The reason 

 why oil colours do not materially change their tint on 

 drying is carefully argued ; and the rationale of Petten- 

 kofer's "regeneration" process for picture-restoring is 

 given. Chapter IX. sums up the indisputable evidence 

 for regarding red, blue (or violet), and green, and not red, 

 blue, and yellow, as the three fundamental colours, and 

 later on is discussed the reason why a greater luminosity 

 is obtained in mixing two colours optically, or by laying 

 them side by side in minute touches, than is obtained by 

 laying them over one another or by mixing them on the 

 palette ; and the author adds no less truly than con- 

 cisely : "every mixture of pigments on the painter's 

 palette is a stride toward blackness.'" 



We can commend the volume to the notice of all who 

 study colour, whether from an aesthetic or a scientific 

 point of view. Silvanus P.Thompson 



OUR BOOK SHELF 



Zeitschrift fiir das chemische Grossgewerbe. Knrzcr 

 Bericht iiber die Fortschritte der chemischen Gioss- 

 industrie. In Vierteljahres-heften, iii. Jahrgang. 

 Unter Mitwirkung angesehener Technologen und 

 Techniker. Herausgegeben von Jul. Post. (Berlin : 

 Verlag von Robert Oppenheim, 1879.) 

 This volume is the third issue of an Annual Report of 

 Chemical Technology in Europe and America, published 



