August 2, 1897.] 



KNOWLEDGE. 



177 



"11 

 illtlSTRATED MAGAZINE 



HERAT 



LONDON : AUGUST 2, 1897. 



CONTENTS. 



Photography in Natural Colours. By ir. Sxowdex 



Wabd, F.K.P.S ' 177 



Ancient Volcanoes of Great Britain. (Ilbisfrafed) ... 179 



The Pedigree of the Cat. Bt E. Ltdekkee, B.A., F.R.S. 181 



Mill and Tennyson 183 



Wind as an Aid to Flight. By F. "\V. Headlet, M.A. ... 183 

 English Medals.— II. Bv G. F. Hill, M.A. {Illustrated) 



{Plate) .".. 18-1 



Science Notes 187 



Letters :— "W. H. S. Monck ; Or- E. Lat alette, B.Sc. ; 



SiLVANus P. Thompson; Chables Atkinson 187 



Notices of Books 190 



Short Notices ... ... .. .. ... ... 192 



Books Received ... ... ... ... .. ... 193 



Astronomical Photography.— I. The Conditions which 

 Determine the Limiting Time of Exposure. By 

 F. L. O. Wadswoeth ... ..".193 



The Metamorphosis of a Dragon Fly, By Rev. A. East. 



{Illustrated) 195 



The Ministry of Leaves. Bv Rev. Alex. S. AVilson, 



M.A., B.Se. ■. 197 



The Face of the Sky for August, By H. Sadleb, F.R.A.S. 199 



Chess Column. By C. D. Looock, B.A.Oxon 199 



PHOTOGRAPHY IN NATURAL COLOURS. 



By H. Sno^t)en Ward, ¥.B..'?.^.,Editor o/" The Photogram." 



LIKE the cry of " Wolf ! wolf ! " raised by the 

 thoughtless shepherd, the announcement that 

 " photography in natural colours " is at last 

 I discovered has been so often made that all men 

 who know aught of photography are apt to shake 

 their heads in graver doubt when each new claimant 

 comes. Like "psychic" photography, the photography 

 of colour has been so largely the subject of fraud and 

 misrepresentation that even an honest worker must expect 

 to be met with scepticism — especially when he makes a 

 mystery of his methods, and talk of largely capitalized 

 syndicates is in the air. This is the case vrith the latest 

 discoverers ; and though one of them has succeeded in 

 inducing Sir Henry Trueman Wood and Captain Abney to 

 vouch for the apparent accuracy of his statements, neither 

 of these gentlemen knows the materials employed to pro- 

 duce the effects that have been shown. 



In briefly dealing with this subject I am bound to give 

 a few words to its earlier history, but will confine my 

 survey entirely to the direct or purely photographic 

 methods. The indirect or three-colour method, by analysis 

 and subsequent synthesis of the light-colour effects, is well 

 known as a practical and commercial success. The earliest 

 recorded observations of photography in natural colours 

 are of much earlier date than the invention of photography 



itself, for while the Daguerreotype was not announced 

 until 1839, the principle of direct colour photography was 

 made public by Scheele and Senebier (1777-1782). 



These workers discovered that chloride of silver deposited 

 on a smooth surface was darkened by the action of light ; 

 but they also went much further, and found that if the 

 light-coloured chloride were exposed to a spectrum of 

 white light the colouring of the silver salt bore consider- 

 able resemblance to the colours of the spectrum by which 

 it was produced. From that day to this the selective 

 colouring of silver chloride has been the basis of many 

 attempts to perfect photography in natural colours. 

 Seebeck, of Jena, brought the subject prominently before 

 the public in 1810 ; while Ritter,Wolla3ton, (Sir) Humphry 

 Davy, and Thomas Wedgwood, all worked upon and 

 reported their experiments in 1801-2. The four last 

 named applied their energies mainly, if not entirely, to 

 the darkening effect, without regard to colour ; but 

 the difficulties were the same in regard to both 

 branches of the subject, and the main difference is that 

 although the fundamental difficulty of " fixing " the image 

 has been overcome in ordinary photography, it has 

 remained insurmountable in the colour work. When once 

 it had been found that silver chloride was changed from 

 (practically) white to (practically) black by the action of 

 light, it was a simple matter to see that by shielding 

 a portion of the surface behind a stencil, a silhouette 

 portrait, or a fern-leaf, a picture of the shield in white 

 upon black, would be obtained. Working with leaves, it 

 would soon be apparent that great delicacy and gradation 

 of the darkening effect was obtainable, for while the ribs 

 and veins of the leaf would be represented by white, its 

 thinner parts were distinctly but faintly tinted. Here was 

 the germ of a very beautiful decorative art, even before the 

 camera method was suggested ; but the difficulty remained 

 that if the picture were examined or exposed in daylight 

 the fainter portions at once began to be tinted, until 

 gradually the whole sheet became one colour, and the 

 picture was lost. For a long time no method of prevent- 

 ing this was discovered, but eventually a solvent was 

 found which would attack and dissolve the silver chloride, 

 but which would not affect the salt in the dark state 

 to which it was reduced by the light's action. Hence, 

 after printing under a leaf, the uncoloured silver chloride 

 could be dissolved out and the picture remained as a 

 permanent silhouette. 



It might seem as if this method ought to apply to the 

 coloured as fully as to the monochrome image, but this is 

 not found possible m practice. In one case there is a 

 definite chemical change, reducing the silver from the 

 chloride to the metallic state. In the other there is a 

 change too subtle for our present chemical and physical 

 knowledge, so that we cannot expect success by this 

 method until science has progressed considerably. By 

 far the most exhaustive scientific work upon the photo- 

 sensitive salts of silver, especially upon their colours, 

 is that carried on by the late M. Carey Lea, who saw no 

 prospect of success as a result. 



To return to the earlier days : Sir John Herschel made 

 reports upon his work in 1819, and in 1841 he expressed 

 before the British Association the opinion that his experi- 

 ments might lead to the production of naturally coloured 

 photograms. Eobert Hunt, working from 1810-43, pub- 

 lished his results in 1844, under the title of " Researches 

 on Light." In 1848, Edmond Becquerel produced some 

 exceedingly sensitive silver surfaces, on which he made 

 pictures in colour, not only of the spectrum, but also of 

 natural objects — brightly dressed dolls, etc. This worker 

 is said to have rendered some of his " heliochromes "per- 



