October 6, 1904] 



NA TURE 



555 



" plnachrome," both these being cyaiiine derivatives. 

 Dr. E. Konig, who has investigated the comparative 

 merits of these three sensitisers, states that if the sensi- 

 tiveness conferred by ethyl red to red light is regarded 

 as 100, " orthochrome T " gives a sensitiveness of i6o 

 to 180, and " pinachrome " 450 to 500. But the density 

 that the best of these sensitisers gives on development 

 in the red and green of the spectrum is not propor- 

 tional to the luminosity of these colours, therefore the 

 exposure for these colours has to be longer than for the 

 blue, but only about three times as long. 



Theoretically perfect colour screens or filters are 

 therefore useless, because a perfect plate, so far as the 

 interpretation of colour is concerned, has not yet been 

 produced. The colour screens and plates must be 

 tested together, and for this purpose it is necessary to 

 have recourse to tlie spectroscope, making photo- 

 graphs, <i: > : i '1 the various conditioiT-. Rul 



that should be obtained when this is photographed, 

 using the given plate and each of the three colour 

 screens in turn. To facilitate the use of the chart, an 

 extra copy is provided in a pocket on the cover of the 

 book. A grey scale of different shades, made on 

 platinum printing paper, is exposed and developed wit;h 

 the colour chart, and the three prints should show this 

 grey scale alike, when the differences due to the 

 colour screens should be as shown in the three prints 

 supplied. Chapm.an Jones. 



(J^o be continued.) 



REFLEXIONS IN WATER.' 



'T^ HERE are few studies more fascinating than 



^ that of the reflexions formed naturally in *:he 



sea, and in rivers and lakes. In the first place, this 



-.IikI- i- iii'MilIv jiii-ii 1 ''I ihe open air; further, 



-Old Harbour Side, Scarborough. Fn 



' Light and Water.' 



spectroscopic results are so liable to deceive observers 

 who are nut thoroughly accustomed to such work that 

 less discriminative methods of testing arc generally pre- 

 ferred. The colour sensitometric methods that Sir 

 William .Abney has done so much to perfect are often 

 employed for this purpose. A series of small pieces of 

 suitably coloured material are arranged in such a 

 manner that when the plate is exposed through its 

 screen and this sensitometer a definite and easily recog- 

 nisable result will be obtained if the plate and screen are 

 mutually correct. Or the colours may be arranged on 

 a rotating disc concentrically with a grey produced by 

 the mixture of definite proportions of black and white, 

 so that the colour and the grey will give an equal 

 density in a photograph of it taken in the camera 

 through the screen on the plate. Colonel Hiibl, in the 

 volume above referred to, gives a colour chart that con- 

 sists of small patches of nine pigments, with the results 



NO. 1823, VOL 70] 



Ihe effects observed are often of surprising beauty; 

 and lastly, most, if not all, of the phenomena 

 observed can be explained in accordance with a few 

 simple principles, so that it is possible for almost 

 anyone possessing a trained faculty of observation to 

 add to our knowledge in this direction. In writing 

 a book on reflexions in water. Sir Montagu Pollock 

 has entered an almost untrodden region withm the 

 borders of both art and science; with the exception 

 of some passages in the works of Mr. Ruskin, it 

 would be difficult to refer to any other work dealing 

 with the same subject. It is no small accomplish- 

 ment to produce a book in which so many intricate 

 effects are traced to their causes, using language 



' Light and Water : a Study of Reflexion and < 

 Sea." By Sir Montagu Pollock, Bart. Pp. x 

 zS cxplan; tjry figures. (London : Gtorge Bell i 



n River, Lake, 

 with 39 plates 

 s, 1903. J Price 



