December 31, 1891] 



NA TURE 



195 



compare viscosities has always been the choice of suitable 

 temperatures of comparison — temperatures at which the 

 substances are in comparable conditions. Reference to 

 the original papers shows that the temperature of observa- 

 tion in the examples given was uniformly 25". Whether 

 under such conditions the viscosities obtained are com- 

 parable is open to question, and this point should have 

 been discussed before any stress was put upon the figures. 

 Such details as these ought surely to have been noted as 

 necessary accompaniments of the experimental results. 



In other directions the same tendency to omit essential 

 particulars is traceable. In describing the series of 

 operations whereby van 't Hoff was enabled to apply 

 thermodynamics to solutions, it is not shown that the 

 cycle, as conceived by him, is a reversible one. The 

 whole practical utility of the process depends on its 

 reversibility, for then only does the second law of thermo- 

 dynamics apply. In connection with this point it is not 

 obvious why the translator should prefer the term "re- 

 versible cyclical process " to the time-honoured and 

 compact " reversible cycle." The use, too, of the shortened 

 " cyclical process " as the equivalent of " reversible cycle " 

 is inaccurate. 



No doubt the incompleteness mentioned is due to the 

 eftbrt made by the author to make the most of his space. 

 In some cases, however, space might be gained. For 

 example, it is surely excessive to give two pages to Voit's 

 method of obtaining diffusion constants, if it led to results 

 which were "quite erroneous" ; or to devote three pages 

 to Planck's deduction of the vapour pressure of dilute 

 solutions, if the fundamental thermodynamical equations 

 are assumed. 



Several points which require alteration may be sum- 

 marized here. On p. 7, van der Waals's equation is given 

 wrongly, a bracket being omitted, b in the equation is 

 four times the volume of the molecules, not the volume 

 of the molecules, as stated on pp. 7 and 34. No definite 

 mention is made of what the ordinates and abscissae are 

 in the diagrams on pp. 66 and 67. On p. 70 " differen- 

 tiating for T" would usually be "differentiating with respect 

 to T." " Narrower " should be " wider " on p. 97, line 1 5. 

 The expression " 200 grams capacity" is used on p. 118. 

 On p. 136, "square root" should be "square." On pp. 

 186 and 187, l^'lf is written for l-f\j'. 



should be \lv. On p. 238, ^ is 



b-'&c 

 I -c ■ 



Mr. Pattison Muir has evidently attempted to give the 

 sense of the original, without confining himself to a 

 literal translation. He has succeeded in m.akinga read- 

 able book, although in one or two instances, as in the 

 account of magnetic rotation, the meaning is slightly 

 obscure. 



A careful study of this the latest addition to the litera- 

 ture on solution will, we think, confirm what to many has 

 been all along apparent — that solution is in the highest 

 degree a complex process, and that the physical theory 

 errs in treating it as being altogether too simple. Despite 

 the success of this theory, which by establishing a striking 

 analogy has admittedly done much in giving a fresh im- 

 petus to investigation, the mechanism of the process is 

 still hidden. The attitude assumed by the upholders of 

 NO. I 157, VOL. 45] 



On p. 237, ijb 

 given instead of 



the physical theory, whereby the presence of the solvent 

 is practically ignored, and analogy regarded as identity, 

 must, of necessity, lead to misconception. 



Much more work must be done, and, whatever happensr 

 more attention paid to the function of the solvent, before 

 any adequate theory of solution is possible. 



J . W. R. 



COLOUR BLINDNESS. 

 Colour Blindness and Colour Perception. By F. \V. 

 Edridge Green, M.D., F.G.S. (London : Kegan Paul, 

 Trench, Triibner, and Co., 1891.) 



IN a work with this title one naturally expects to find 

 that such recognized authorities on the theory of 

 colour perception as Young and Helmholtz are treated 

 with the respect due to their labours and researches. 

 The writer, however, not only refuses to pay homage at 

 the shrine of such masters of natural philosophy, but 

 deliberately devotes a considerable portion of his work to 

 an exposure of the "fallacy of the Young -Helmholtz 

 theory." The preface informs us that the book has been 

 written for the benefit of those who may have to test for 

 colour blindness. To such it will hardly be a recom- 

 mendation to learn that the theories of Young and Helm- 

 holtz are mere fallacies, and that the tests for colour 

 blindness as instituted by Prof. Holmgren are not worthy 

 of the name. The question, of course, arises. What 

 theory are we to adopt relative to colour perception when 

 we have surrendered our allegiance to the theories which 

 Mr. Green denounces ? Ths author answers this query 

 for us by propounding his own doctrine — " an application 

 of the theory of psycho-physical perception, described in 

 my book on ' Memory,' to the phenomena of colour blind- 

 ness and colour perception." The arguments in support 

 of this theory are based upon the examination of some 

 116 colour-blind persons, not an over-large number of 

 cases to generalize from, especially when we learn some- 

 thing of the method pursued in the examination. In- 

 formation afforded by the colour-blind themselves is one 

 of the chief sources of Mr. Green's knowledge respecting 

 colour blindness. He states that he has derived much 

 valuable information from colour-blind persons relating 

 to facts concerning their colour perception. We question 

 much the trustworthiness of data acquired by interview- 

 ing colour-blinds as to the phases of their visual infirmity. 

 Yet Mr. Green characterizes this information as trust- 

 worthy, and alludes to it as " definite facts of colour- 

 blindness, to which any future theory must conform." 

 Many writers on colour bhndness have stated that 

 naming colours is a useless and misleading method ot 

 examination, because the colour-blind must use the con- 

 ventional colour names and use them at random. But 

 this reasoning, we are told, is a fallacy, because the 

 colour-blind do not name colours at random, but in 

 accordance with their ideas of colour ! Such is the 

 language in which the author disposes of the " fallacy '' 

 of Holmgren's wool test. Equally illogical is another of 

 his conclusions : " If, as some persons have said, testing 

 by colour names is useless, then the whole series of 

 colour names is useless." 



Prof. Holmgren, it is admitted, has done good 

 service in bringing the subject of colour blindness 



