June, 1905.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



137 



Photography- 

 Pure and Applied. 



By Chapman Jones, F.I.C, F.C.S., &c. 



Tlie selection of a pnniing process. — In scientific work 

 one is often satisfied with the production of the nega- 

 tive, and sometimes rightly so, but for demonstration, 

 reproduction with letterpress, storing for convenient 

 reference, and in other cases, it may be desirable, if 

 not necessary, to make prints. In one sense a print 

 must always be inferior to the negative because it is 

 a stage, further from the original, but it may be practi- 

 cally far superior because it gives the photographer 

 further opportunity for making more conspicuous the 

 very matter that he wishes to investigate or demon- 

 strate. In the choice of a printing process there are 

 more possibilities than are generally recognised. 



There are now to be obtained silver printing-out 

 papers specially prepared for giving vigorous prints 

 from poor and flat negatives, and by the use of them 

 important detail in a photo-micrograph or a spectrum 

 photograph that is feebly represented in the negative 

 may be made much more conspicuous, .'\mong de- 

 velopment papers, slow bromide or " gas-light " 

 papers are specially suitable for this purpose. A 

 smooth surface should always be selected, and if it is 

 made more shiny still by drying it on a sheet of ebonite 

 (or glass or ferrotype iron), the detail will show still 

 more markedly. It may be that in a print so obtained 

 much of the other parts of the subject will be lost in 

 obscurity, but then it is easy to prepare another print 

 on an ordinary paper, making the best of the subject 

 as a whole, and to show this with the special print of 

 the particular part that needs emphasis. 



If it is desired to show the general characters of a 

 subject without special emphasis of detail, as may be 

 the case with photographs of some geological sub- 

 jects, a rougher surfaced print is an advantage. A 

 matt-surfaced bromide print, or a print on a paper that 

 has no layer of medium on it (gelatine or albumen), 

 such as platinotype paper and some silver papers, will 

 serve this purpose. Here the detail will not be lost, 

 but it will be less obtrusive. 



If permanency is the chief desideratum, there are 

 three processes that specially come to mind, namely, 

 platinum and carbon printing, and the production of 

 enamels. Of these, undoubtedly the most convenient 

 for those who do not make a business of photography 

 is printing in platinum, and, although it is making 

 rather a fine distinction to compare the probable last- 

 ing properties of these three kinds of photographs, I 

 think that a platinum print would probably out-live 

 the others. If it were my duty to prepare photographic 

 records for the express purpose of being in usable 

 condition a thousand years hence, I should be inclined 

 to prepare prints by these three processes, unless the 

 subject was too large for making an enamelled plate 

 from it, and then I should not much regret having to 

 rely on the other two. But if I omitted to prepare 

 platinum prints, I should feel that I had not been 

 faithful to my trust. If a platinum print is not brilliant 

 enough to clearly show the detail to which attention is 

 to be directed, it may be waxed, a process that used 

 to be in vogue years ago in connection with silver 

 prints, but is rarely used now. For this purpose white 

 wax is melted with turpentine in such proportion that 

 the mixture, when cold, is of the consistency of a thin 



pomatum. This is applied to the surface of the 

 mounted print by means of a small flannel pad with a 

 light polisiiing movement similar to that adopted when 

 "French polishing" wood-work. 



Gelatine v. Collodion, etc. — It has sometimes been 

 deplored, for the sake of experimental rather than 

 practical photography, that collodion has given place 

 to gelatine as the vehicle of the sensitive salt. Gelatine 

 is supposed to be comple-v, variable, and uncertain, 

 and no doubt it justifies its reputation, but whether 

 the collodion film is either more simple, stable, and 

 reliable is open to considerable doubt. Those who 

 iiave stored both gelatine and soluble guncotton will 

 know that the former appears to remain unchanged 

 indefinitely, while the latter cannot be preserved in a 

 glass bottle for very long, because of the continual 

 evolution of acid vapours that must be allowed to 

 escape, and that if stored as is usual in paper lined tin 

 canisters or cardboard boxes, the paper gets rotten 

 and the tin corroded. It is too often taken for granted 

 that guncotton is merely cellulose nitrate, and that the 

 sulphuric acid used with the nitric acid in its prepara- 

 tion merely facilitates the action of the nitric acid on 

 the cotton, perhaps chiefly by its dehydrating action. 

 But it has long been known that sulphuric acid has a 

 specific action of its own upon cotton, and Messrs. 

 Napier Hake and R. J. Lewis have recently shown 

 [Jnl. Soc. Chem. Ind., 29th April, 1905) that cellulose 

 sulphates are generally, and probably always, formed 

 in small quantities in the preparation of guncotton, and 

 that they often, if they do not always, remain in the 

 finished product, and are an element of instability. 

 This investigation refers to the guncotton of warfare, 

 and photographers who refer to the paper should bear 

 in mind that soluble guncotton or pyroxyline is pre- 

 pared with far less care than the other. 



Experimentalists who want a pure sensitive film free 

 from the uncertainties of either gelatine or collodion 

 have sometimes regarded the daguerreotype process as 

 very advantageous. But even here there are uncer- 

 tainties, for General Waterhouse has shown that an 

 ordinary clean silver surface is sensitive to light, while 

 if thoroughly cleaned by heating and treatment with 

 acid it becomes insensitive. These is little doubt that 

 whatever support or medium is used for the sensitive 

 salt, its character must be taken into account in in- 

 vestigational work, and that none of those hitherto 

 shown to be available can be regarded as inert. But 

 this is no justification of the extreme view that has 

 sometimes been expressed to the effect that the sensi- 

 tive substance in ordinary plates is not silver bromide, 

 but a product of the action or combination that has 

 taken place between it and the gelatine. 



Radiation or Emanation.— The. fact that many sub- 

 stances give off something, whether a radiation or a 

 gaseous emanation, that produces the developable con- 

 dition in gelatino-bromide plates is being gradually ex- 

 tended. The latest additions to the list of " active '' 

 substances are mercuric cyanide, mercuric chloride, a 

 few other mercury salts, and a compound of mercuric 

 cyanide with phe'nylhydrazine. Metallic mercurv-vvas 

 found to be quite inactive, as Dr. Russell stated it to 

 be some years ago. Messrs. R. de J. F. Struthers and 

 J. E. Marsh have obtained these results, and further 

 details concerning them will be found in their paper 

 published in the Journal of the Chemical Society for 

 A pril (p. 377)- 



We regret that the word actinism in two places i 

 spelt activism-contrary to author's copy. 



the May issue appeared 



