Nov. 20, 1884] 



NA TURE 



61 



volving axis, are suppressed for the sake of clearness- 

 Under the condition of rapid rotation both disks ap- 

 pear to be coloured a uniform grey. 



Thus in a single experiment is demonstrated (1) that 

 blue and yellow are complementary colours, (2) that 

 particular tones of blue and yellow produce by mixture 

 of retinal impressions a white of low tint — in fact, by 

 measuring the sectors composing the outer annulus, a 

 white of 2-9ths the intensity of that of the annulus, which 

 is produced by barium sulphate. Other binary combina- 

 tions will be found to produce similar results, e.g. red and 

 blue-green, violet and yellow-green. In fact given any 

 hue, a second may be formed by means of this apparatus, 

 such that the combination of the retinal impressions proper 

 to each shall produce the sensation of white, the degree 

 of this sensation varying with the tone of the constituents 

 of the combination. 



From the experimental investigation of complementary 

 hues the definition of intensity is readily deduced. In 

 the sectors of the plate we notice equality of area. Had 

 we taken a fuller yellow of the same hue, the grey pro- 

 duced with the blue sector of equal area would have 

 shown a yellow cast, and to restore the neutral grey or 

 low white we must increase the area of the blue at the 

 expense of the yellow. The relative intensity of comple- 

 mentary hues is thus defined to be the reverse of the 

 sectors necessary to produce neutrality of hue. 



The use of the particular yellow pigment of Fig. I, 

 coloured in the original chromate of lead, is dictated by 

 the lowness of tints of all our blue pigments ; the purest 

 of our ultramarines, smalts, and aniline blues do not 

 possess one-third the intensity of chromate of lead ; and 

 the same is true of the greens and violets. 



The study of complementary colours leads directly to 

 the discussion of the basis of this phenomenon, whether, 

 i.e. it is physical or physiological? It is in this depart- 

 ment of the subject that confusion of ideas has longest 

 persisted. Although it was pointed out by M. Plateau as 

 long ago as 1 829 that the mixture of co\ouring->/iatters and 

 of co\our-se//satia/is are distinct phenomena, the classical 

 experiment of Muschenbroek, dating from 1762, is still 

 retained by lecturers and text-books, together with erro- 

 neous interpretations. Newton himself fell into the same 

 error in his discussion of the recombination of the spectral 

 colours. The author puts the matter in the clearest light 

 by pointing out that there are a number of mixtures pro- 

 ducing the sensation of white light, — that psychological 

 identity, therefore, is no criterion of physical identity. 



The distinction is perhaps most clearly demonstrated 

 by a plate of figures representing the superposed disks at 

 rest and in motion. The outer annulus is composed of 

 alternate and equal sectors of blue and yellow, the inner 

 disk being coloured with a mixture of blue and yellow 

 pigments in equal proportions. The distinction in ap- 

 pearance produced by motion affords the clearest demon- 

 stration of the point in question (Figs. 3 and 4). 



The next portion of the treatise is devoted to the study 

 of mixtures of colours, i.e. colour-sensations, which are not 

 complementary. The more important results are those 

 obtained in the so-called '' degradation " of pigments. 

 Such pigments, for instance, when applied to a white 

 surface, will be more or less mixed with white, i.e. the 

 sensations of white will be more or less conjoined with 

 that of the pigment hue, as the quantity of pigment per 

 unit of surface is less. The author reproduces series of 

 such tones, in the case of Prussian blue and chrome 

 yellow, together with their respective complementaries. 

 In both cases it is found that the progression is accom- 

 panied by an alteration in hue, the fuller tones being dis- 

 tinctly redder. It is clear, therefore, that to construct a 

 scale or gamut of tones with any given pigment, in order 

 that this shall have an aesthetic or standard value, each 

 tone must be referred to the same complementary, and 

 the tones due to the pigment alone will need correction 



in accordance with their demonstrated imperfections, i.e. 

 departures from the standards determined by the method 

 of physiological comparison. 



The author has very carefully compared such scales of 

 tones with the purely arbitrary scales of M. Chevreul, and 

 has found the differences to be considerable. Such indeed 

 might be inferred indirectly from M. Chevreul's definition 

 of " the tones of a colour " ; they are, according to him, 

 " the different degrees of intensity of which a colour is 

 susceptible according as the substance by which it is pro- 

 duced {represents) is pure or mixed with white." 



A comparison of colour combinations harmonised ac- 

 cording to the two systems, will show the aesthetic supe- 

 riority of the physiological method, judged, that is, by 

 the much abused arbiter, taste. It is unnecessary farther 

 to insist upon the practical importance of such con- 

 clusions. It will doubtless have been already appreciated 

 on the part of the reader that the confusion of ide >s which 

 it is the object of this treatise to eliminate cannot have 

 remained without influence upon the education of the 

 eye ; nor can he fail to see that the training involved in 

 the practice of the author's experimental method is a 

 valuable aesthetic discipline, as well as a precise study 

 of colour relationships. 



We have attempted to give an idea of the difference in 

 appearance of the disks by lines on a white surface. 



THE LA TE FERDINAND VON HOCHSTETTER 

 HTHE numerous friends and admirers of the late Dr. 

 * Ferdinand von Hochstetter in Europe and Austral- 

 asia have to thank his old associate, Dr. Julius von Haast, 

 for a graceful tribute paid to his memory, which takes 

 the form of a sympathetic biographical notice published 

 towards the end of last August at Chnstchurch, New 

 Zealand. The memoir, which is accompanied by two 

 portraits, from a lithograph and a photograph showing 

 the distinguished naturalist in his twenty-ninth and fiftieth 

 years respectively, is taken for his early career partly 

 from an account in Brockhaus's " Conversations Lexicon," 

 and for the period since the two friends first met at Auck- 

 land, N.Z., in 1858, from Hochstetter s writings and private 

 correspondence. Born on April 30, 1S29, at Esslingen, 

 Wurtemberg, the future naturalist was at first intended 

 for the Church by his father, Prof. Christian Ferdinand 

 Hochstetter, chief pastor of that town, and himself a 

 botanist of no mean repute. But in the seminary of 

 Maulbronn near Tubingen, his love of science, implanted 

 in the paternal home, grew so strong that, besides theo- 

 logy, he applied himself with great zeal to the study of 

 mineralogy, palaeontology, and geology. After taking his 

 degree of Doctor Philosophiae in 1S52 he seems to have 

 finally made choice of a scientific career, and in 1853 

 found employment on the Geological Survey of the Aus- 

 trian Empire, soon after receiving the appointment of 

 Chief Geologist for the Bohemian Section. His reports 

 on the geology of the Boehmer Wald were so highly 

 appreciated that he was selected in 1857 as geologist of 

 the Novara Expedition, which brought him to Auckland 

 on December 22, 1858. Here his services were at once 

 secured by the Government, and with the reluctant con- 

 sent of the Commodore of the Novara he accepted an 

 engagement of eight months to examine the geology, 

 physical features, and natural history of New Zealand. 

 During this period he made extensive topographical and 

 geological surveys of the provinces of Auckland and 

 Nelson, the results of which were embodied in his stand- 

 ard work, " Neu Seeland," published in 1863, followed in 

 1867 by the greatly enlarged English edition dedicated to 

 the Queen. Soon after his return to Europe he was 

 appointed Professor of Geology and Mineralogy in the 

 Technical University of Vienna, and after a visit of some 

 months to England in 1S60 he settled permanently in 

 the Austrian capital, where, in April 1861, he married 



