NATURE 



[July 23, 1874 



change of colour which hght of a certain wave-length 

 undergoes when the intensity diminishes. Prof, von 

 Bezold uses this curious fact to explain the peculiar 

 colours seen in a landscape when viewed by moonlight, 

 although the light reflected by the moon is identical in 

 composition with simlight. 



In his account of the elementary principles of optics the 

 author abandons the old method of dividing vibrations 

 into heat rays, light rays, and actinic rays. We note this 

 point as it is one which must soon play an important part 

 in physics and will doubtless provoke much discussion. 

 The author seems to prefer the following method of 

 viewing the facts to the old one : — A body absorbs a 

 certain class of rays peculiar to itself ; whether these 

 rays are converted into heat or into chemically active rays 

 depends upon the peculiar properties of the body. In 

 order, however, to include in this statement all the facts 

 included in the old division, we must add that, as a rule, 

 bodies absorbing the ultra-violet rays are thereby ren- 

 dered more chemically active, and, as a rule, bodies 

 absorbing the red are thereby heated. This method of 

 koking at the matter seems to us to be the one most 

 closely agreeing with the facts. Prof, von Bezold gives, 

 as a proof that the red rays may be chemically active, the 

 fact that, as the green colouring matter of leaves absorbs 

 the red end of the spectrum as well as the blue, the red 

 rays alone are sufficient to sustain life in the plant. He 

 might have referred to the recent discovery of Vogel, 

 who photographed the red end of the spectrum by 

 mixing a red colouring matter with bromide of silver ; 

 and, on the other hand, to the fact observed by Budde, 

 that chlorine is heated by the ultra-violet rays. The 

 third chapter contains a short and clear abstract of 

 recent researches on compound and primary colours. We 

 would call attention specially to the passage in this chap- 

 ter on colour and sound, in which the author refers to the 

 influence of dwelling too much on the analogy between 

 sound and light. Analogies are a very dangerous help to 

 teachers, and are used by far too often. It requires at 

 least a partial knowledge of the subject to see where the 

 analogy begins and where it ends. Students generally 

 either do not see where the tnalogy really lies, or want to 

 carry it too far ; a good many erroneous nations are 

 thereby acquired. 



The most interesting chapter in the book, however, is 

 the one on Contrast of Colours ; the examples are well 

 chosen, and the coloured illustrations in the accompany- 

 ing plates are in all cases convincing. The author shows 

 with great success how little we may trust our own eyes 

 as regards colour, and how difficult and even impossible 

 it is to form a correct judgment of the relative darkness 

 of two shaded fields, so long as they are not on the same 

 ground. 



The last chapter, which treats of the combination of 

 colours, is necessarily the least complete ; it shows, how- 

 ever, that the application of the theory to the arts has 

 fairly begun. It has already been said that this beginning 

 does not justify us in demanding from painters obedience 

 to rules which have not been proved to be valid without 

 exception. It may be easy to discover the application of 

 these rules in acknowledged masterpieces, and yet be 

 difficult to state them in such an exhaustive way that 

 compliance with them will in all cases lead to perfect har- 



mony. So long as this is not done it must not be expected 

 that the painter will derive substantial help from the theory 

 of colours. Arthur Schuster 



OUR BOOK SHELF 



Illuslrations of the Principal Natural Orders of tlie 

 Vegetable Kingdom. Prepared for the Science and 

 Art Department of the Council of Education. By 

 Prof. Oliver, F.R.S., F.L.S. (London, Chapman and 

 Hall, 1874.) 



Few books published of late years will be of greater 

 practical value to the botanical teacher or student than 

 this. The want has long been painfully felt of a work 

 which will give in as few words as possible the salient 

 characters of each of the more important natural orders, 

 unencumbered by minutia; of structure which concern 

 only the more advanced student. This want we have 

 here most admirably supplied, not only by 150 pages of 

 text, but by upwards of 100 plates, which present in the 

 most lucid form a representation (plain or coloured, as 

 may be preferred) of a section and '' diagram " of a flower 

 belonging to many orders, together with a drawing of the 

 fruit, seed, or other organ the structure of which is of special 

 importance. The very comprehensive title of the work 

 might, unless the contrary is pointed out, lead to a little 

 disappointment, when it is found that the descriptions, and 

 still more exclusively the plates, refer almost entirely to 

 the more important ^zc/v/iiz// orders ; very brief accounts, 

 or in some cases none at all, being given of such remark- 

 able extra-European groups as the Cycadea;, Gnetacea:, 

 Proteaceas, Bignoniacere, Piperaces, and others. As 

 far as European botany is concerned, we cannot conceive 

 that the work could have been better carried out. The 

 plan which has been adopted of treating separately groups 

 which are united together into a single order in our more 

 advanced text-books — as for instance Fumariacere as dis- 

 tinct from Papaveracese ; Oxalidea; and Tropsolacese 

 from GeraniacCc'E, and Droseracese from Saxifragacea:- - 

 seems to us altogether commendable in a work designed 

 especially for beginners. There has long been felt a desire 

 that in text-books of botany the morphological and phy- 

 siological portion should be divorced from the systematic 

 and descriptive. We trust that in future this may be 

 carried out, and that writers of text-books will confine 

 themselves to the former branch, leaving the student to 

 gain his elementary knowledge of the latter branch from 

 special works like the one before us. A. W. B. 



LETTERS TO THE EDITOR 



[The Editor does not hold himself resfoiisihle for opinions expressed 

 by liis. correspondents. No notice is taken of anonymous 

 communications i^ 



Photographic Irradiation 



In answer to Mr. Ranyard (Nature, vol. x. p. 205), I have 

 to state that the opaque bar in my experiments was placed as 

 close to the collodion as possible without touching it, not farther 

 than '01 in. from it, and that there were no photographic traces of 

 diffraction bands. 



Allow me now to suggest a possible explanation of the dif- 

 ferent results given by Mr. Kanyard's and my own experiments. 

 One important difference in the arrangement of the two experi- 

 ments was, that in the one case the opaque bar was in contact 

 with the collodion, and in the other case it was placed at a very 

 short distance from it. In the experiments with the bar in con- 

 tact with the collodion, the nitrate of silver solution on the 

 surface of the jilatc would not form a true plane but would be 

 curved upwards at the edge of the bar ; and further, this curve 

 would not be regular, but would have irregularities correspond- 

 ing to every irregularity in the edge uf the bar. This irregular 

 curved fluid surface would cause irregular refraction of the light 



