October 27. 1923] 



NATURE 



621 



Cycad, Macrozamia spiralis. He has found urease 

 also in the seeds of Abrus precatorius." 



The earlier experiments referred to were my own. 

 Mr. Benjamin had assisted Mr. Horton and me in 

 our work on urease, published early in 191 3. He 

 was a young Australian and he undertook the observa- 

 tions, on his return, at my request. When in Java, 

 in September 1914, I had the opportunity, at the 

 Buitenzorg gardens, of testing fresh Abrus seeds and of 

 confirming Benjamin's result. I may say, the amount 

 present is small, in no way comparable with that in 

 Soja beans. 



To me the presence of urease in the nodules is 

 little short of a matter of course — in view of their 

 " ammonicity." The interest of the observation lies 

 in the possible application thereof. 



Urea is foreshadowed as the nitrogenous fertiliser 

 of the future but apparently it has its limitations. 

 All soils, all plants, do not respond to it equally. I 

 was told years ago, that it is particularly good for 

 Pease. Why, Mr. Peasecod ? Probably it is not 

 operative as such but merely as a source of ammonia 

 and must be hydrolysed to make it available. Only 

 soils which contain urease would respond. A clover- 

 sick soil may well be wanting in the organisms which 

 give rise to the nodular growths. 



To be practical — it would seem to be desirable to 

 test the comparative effect of urea on the growth of 

 non-leguminous plants when grown with and without 

 a leguminous plant, such as clover. 



Henry E. Armstrong. 



Colour Vision and Colour Vision Theories. 



In the first of my two recent letters on this subject 

 I selected five of the cases in which Dr. Edridge- 

 Green asserts that the trichromatic theory cannot 

 explain certain phenomena of colour vision ; and I 

 indicated, in each case, the source of his error. In 

 more than one case I gave the full proof. In his 

 reply he took no notice of these proofs except in so 

 far as he seemed to admit their accuracy. But he 

 brought forward three other cases, asserting incom- 

 petence of the trichromatic theory in connexion with 

 them. In my second letter (Nature, September 8) 

 I similarly indicated the oversight involved in each 

 of these three additional assertions. 



I must confess, therefore, to some degree ot surprise 

 that Dr. Edridge-Green, in his letter appearing in 

 Nature of September 29, should say that he will deal 

 with my explanations regarding the competence of 

 the trichromatic theory when I give them. They are 

 already given, and I shall be glad if he will discuss 

 them. To make the matter definite, I invite him to 

 discuss the trichromatic explanation which I have 

 given, in my first letter, of the case of so-called red- 

 blindness with shortening of the spectrum at the red 

 end. The proof is fully given. Another proof, fully 

 given in geometrical terms, is that dealing with the 

 possible diminution of colour sensitiveness by the 

 annulment of one component sensation. 



Instead of discussing any of the eight explanations 

 which I have already given either in full or m outline, 

 Dr. Edridge-Green now points out that he is not 

 alone in regarding the trichromatic theory as in- 

 ade(iuate. Unfortunately, misunderstanding of the 

 theory is too regrettably widespread for the reason 

 which I expressed in my first letter. If any reader 

 who is interested in the matter will refer to the dis- 

 cussion which I have given in my book he may 

 recognise that the statement referred to in Dr. 

 Edridge-Green 's last letter, concerning contrast and 

 colour blindness, is not correct. It cannot be dis- 

 cussed in the scope of a short letter. 



I appreciate Prof. Frank Allen's work greatly. 



NO. 2817, VOL. 112] 



The difficulty to which he refers vanishes, as I am 

 sure he will readily recognise, when the three variables 

 (threshold values) descriptive of non-external action 

 are considered. In fact, in the whole field of contrast, 

 after-images, recurrent images, and inhibition, the 

 trichromatic theory has at its disposal a double set, 

 not a single set, of three variables. Such work as 

 that of Prof. Frank Allen is of great importance in 

 view of the need of a formulation of the threshold 

 values as functions of precedent illumination, time, 

 secondary stimuli, etc. His early work, long ago, 

 led me, in attempting something different, to full 

 recognition of the sufficiency of the trichromatic 

 theory. 



What blindness must have oppressed the mental 

 vision of Helmholtz, " that investigator, worthy of 

 wonder, leaping before his time," if it were true, as 

 Dr. Edridge-Green asserts, that " There is no fact 

 that directly supports the trichromatic theory." 

 Which Helmholtz elaborated so as to fit facts, and 

 used victoriously to predict others ! I know of none 

 that fails to support it. I have studied Dr. Edridge- 

 Green's book very carefully, and I have not found one 

 of his strictures upon the theory with which it was 

 possible to agree. Even Sir William Abney, one of 

 the supporters of the theory, whose experimental 

 work was so admirable, was led to some wrong 

 conclusions through non-perception of some of its 

 possibilities. W. Peddie. 



Dundee, September 29. 



Sexual Physiology. 



In Nature of September i, p. 317, under the 

 heading " Sexual Physiology," a review appeared of 

 the second edition of Dr. Marshall's book " The 

 Physiology of Reproduction." In the course of this 

 notice certain misleading statements are made regard- 

 ing myself. The reviewer, in referring to the chapter 

 of the work dealing with the subject of the fertilisa- 

 tion of the ovum, states, " The least satisfactory part 

 of the book, both as regards arrangement and subject- 

 matter, is, we think, that contributed by Dr. Cresswell 

 Shearer on fertilisation." 



May I point out that I am not the author of this 

 chapter ; while I have revised Dr. Marshall's manu- 

 script, and added a number of notes here and there 

 of minor importance, the two sections of which I am 

 the author are clearly indicated in the footnotes, and 

 I think are sufficiently obvious. In regard to that 

 part of the chapter which has called fortli the special 

 criticism of the reviewer, "The hereditary effects 

 of fertilisation," I am altogether unresponsible, 

 although I completely agree with many of the 

 opinions expressed by Dr. Marshall in this section. 

 As the whole of this paragraph appears almost 

 unaltered in the old edition, it would seem that 

 your reviewer is by no means as familiar with the 

 original work as he would have us believe. 



C. Shearer. 



A footnote to Chapter vi., " Fertilisation," states 

 that this has been " Revised, with numerous addi- 

 tions, by Cresswell Shearer." It was assumed, from 

 this, that Dr. Shearer had taken the chapter as it 

 stood in the first edition and had made himself re- 

 sponsible not only for the numerous additions but 

 also for the whole of the subject-matter of this 

 chapter in the present edition, and for its presenta- 

 tion. That we are not alone in reading this meaning 

 into the footnote is shown by the fact that another 

 reviewer, writing elsewhere, states that " Dr. Cress- 

 well Shearer has written in this edition a most 

 excellent chapter on fertilisation." If Dr. Shearer 

 did revise the chapter, then his objections are but 



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