37B 



NATURE 



{March 14, 1878 



and we shall not say what we thought of them. Let us 

 still be just. If we are to condemn them, let it only be 

 after a trial when they are fresh and good. We have 

 indeed tried them in their native country, pounded up 

 with acorns and mashed into balls by the digger Indians 

 of California ; but then acorns would destroy any dish 

 for civilised food, so that we prefer to leave the question 

 of their culinary merits an open one for some gas- 

 tronomic jury, stipulating only for the right of challenging 

 Mr. Riley, as one of its members, on the score of un- 

 due favour and partiality arising from too intimate an 

 acquaintance and familiarity with the individuals under 

 trial, 



A further contribution to the subject treated of by Mr. 

 Riley has reached us in the shape of the first two 

 Bulletins of the United States Entomological Commis- 

 sion. Andrew Murray 



ABNEY'S TREATISE ON PHOTOGRAPHY 



A Treatise on Photography. . By W. de Wiveleslie Abney, 

 F.R.S. (London : Longmans and Co., 1878.) 



ALL those interested in this most attractive study will 

 welcome Capt. Abney's treatise on photography. 

 Those who wish to become acquainted with the scientific 

 principles on which the practice of photography depends 

 will find in the opening chapters a clear and concise 

 description of the theory of sensitive substances, and of 

 the action of light on ^various compounds, whilst by 

 studying the closing chapters of the volume they will be 

 able to make themselves acquainted with the present state 

 of our knowledge on the important subjects of actino- 

 metry, photo-spectroscopy, and the interesting disco- 

 veries made by the author and others on the sensitiveness 

 of different salts, and the methods employed for obtaining 

 pictures of the various portions of the spectrum. On the 

 other hand, the artist photographer will find ample matter 

 for interest in the chapter in which Capt. Abney most 

 successfully lays down the rules which must guide the 

 production of an artistic picture, pointing out the special 

 difficulties under which the photographer lies in the 

 choice of subjects in order to avoid incongruity or inar- 

 tistic massing of light and shade, and showing the best 

 mode of lighting and arranging the picture by choosing 

 the right point of view for the camera. As an illustra- 

 tion of Capt. Abney's happy style and power of artistic 

 treatment, we may quote the following description of a 

 landscape : — 



" In the next picture, we have the distance, or 

 perhaps more strictly speaking, the middle distance as 

 the point of interest. The horizon line is kept in the 

 weakest part, the centre, of the picture. The trees in the 

 foreground are so grouped that they frame the view with 

 dark masses, relieved by the light foliage of some of the 

 nearer bushes and shrubs. The foreground finishes at a 

 distance of about \ from the bottom. More of it would 

 take away from the value of the middle distance, as it 

 would place it in the weakest part of the picture— viz., 

 centrally ; less of it would have rendered the picture bald, 

 and have cut off part of the deeper shades which are so 

 valuable in giving the effect of distance to the stream 

 beyond. This picture would have been spoilt had the 

 camera been so placed as to give more top foliage, since 

 the bough which now partially crosses the picture at about 

 f the height, would have caused an ugly division, and 

 also the tops of the distant trees, and the sky would have 



appeared. This latter, in views such as that under 

 criticism, is objectionable, as patches of white give the 

 eye an inclination to wander off towards it, and it would 

 have been an insufficient precaution to have printed in 

 clouds from another negative, owing to the difficulty that 

 would exist in subduing at the same time the lights on 

 the leaves of the near trees. As it is, the picture is in 

 pictorial focus. By placing the stream to the right or 

 left, the balance would have been wanting, and its general 

 direction would have been altered to such an extent as to 

 have given a feeling that it was a subsidiary part of the 

 picture instead of an essential." 



Another important section of the work is devoted to the 

 necessary, but unavoidably dry descriptions of the very 

 numerous photographic processes and manipulations now 

 in vogue, of the construction of apparatus, and a state- 

 ment of the general laws of geometrical optics so far as 

 concerns the principles on which the construction and 

 use of photographic lenses depend. On all these subjects 

 we find Capt. Abney's statements clear and concise. 



Then again no book on photography would be com- 

 plete without an explanation of the various processes of 

 photo-lithography and photo-engraving, and accordingly we 

 find a short account of the more important of these inter- 

 esting methods of reproducing photographic effects. To 

 one of these photo-relief printing processes, that dis- 

 covered by Warnerke, with, we believe, the author's co- 

 operation, we would especially draw attention, the 

 picture being remarkable for the beauty and delicacy, as 

 well as for the force and depth of its tones. The details 

 of this process are not yet published ; it cannot, however, 

 be doubted that it is capable of producing the finest effects 

 of a steel or copper-plate engraving. 



It is, however, the scientific side of Capt. Abney's book 

 which will especially interest the readers of Nature. 

 The explanation of the effect of vibration as setting up 

 chemical change in the molecule is clearly set forth in 

 Chapter III. The case in v/hich the atoms are in a stable 

 though verging on an indifferent equilibrium as with the 

 sensitive mixture of chlorine and hydrogen, being well 

 illustrated by the equilibrium of a frustum of a pyramid 

 standing base uppermost on as narrow section of the base 

 as we please. In these cases a very small amount of 

 work is needed to make the systems take up more stable 

 positions. Then " extending our previous illustration, 

 supposing we had a row of such frusta of pyramids, 

 and that it was found that one pellet of a number (all 

 being of equal weight) when striking one frustum with 

 a certain velocity was able to cause it to fall, and also 

 that in every case the accuracy of aim. was undoubted, 

 and that in falling one frustum did not strike its neigh- 

 bour, then at any interval after the commencement of a 

 bombardment the amount of work expended in projecting 

 the pellets could be compared by simply counting the 

 number of frusta which had fallen" (p. 12). The ques- 

 tion of the action of vibrations synchronous with the 

 oscillations of the molecule on the stability of the 

 molecule is next discussed, and the explanation ren- 

 dered clear by a description of Rankine's well-known 

 contrivance of the heavy and light pendulums. The 

 difference between the decomposition of explosives 

 and of bodies einployed for photographic purposes in 

 respect to the nature of the disturbing vibrations is thus 

 pointed out. Explosives are affected by long wave rays, 

 photographic actions as a rule being only set up by waves 



