456 



KNOWLEDGE & SCIENTIEIC NEWS. 



IJlNE, 1906. 



Photography. 



Pure and Applied. 



By Chapman Jones, F.I.C, F.C.S., ice. 



Specially Scnsi/iscd Plates. — A little more than a year 

 a.ifo I referred to the advantaifcs of bathing plates in 

 the prepared dye solution in order to increase their 

 sensiti\eness to yreen, yellow, and red, as compared 

 with the incorporation of the sensitisintf substance with 

 the emul.sion before the phites are coated, and stated 

 that Messrs. Sanger-Shepherd and Co. were [)rcparinii' 

 to issue a bathed plate commercially. The dilliculty, it 

 not the impossibility, of preparintr bathed plates in 

 lari^e quantities that shall be uniform in the various 

 liatches and remain without appreciable chang^e for a 

 sullicient time to render them practically available for 

 general work, has caused this firm to issue an emulsion 

 ^;e^sitised plate instead. The " Sanger-Shepherd " 

 plate is of excellent C|ualify, moderately rapid, and 

 shows a very even sensitiveness to red, green, and 

 yellow, when tested by daylig-ht. In this it is much 

 superior to many plates si)ecially sensitised for red or 

 for the three colours named. The proportion of red 

 sensitiveness to total sensitiveness in the sample I have 

 examined is somewhere ;ibout four times as great as in 

 the plates that Messrs. .Sanger-Shepherd and Co. used 

 to sup[)ly for use in their process of colour photography, 

 anil this indicates a very great improvement. 



Plates bathed with pinachrome and with pinacyanol 

 have recently been put on the market by Messrs. 

 Wratten and W'ainwrightj and the makers state that 

 they are of very great general sensitiveness, and that 

 the sensitiveness of one of them at least to yellow light 

 is almost equal to its sensitiveness to blue light. I 

 have not seen a statement as to the sensitiveness to red 

 of either, thoug'h they are claimed to be specially good 

 in this matter, and presumably with very gocxl reason. 



for those who desire bathed plates without the 

 trouble of treating them, Messrs. Penrose and Co. 

 announce that they are prepared to sensitise plates by 

 bathing to order, using either of the following dyes : — 

 ortiiochrome T for general colour sensitiveness, pina- 

 chrome for general sen.sitiveness with a high speed, 

 pinacyanol for red sensitiveness and high speed, dicya- 

 nine for sensitiveness to red extending to the less re- 

 trangible colour, and also, for green sensitiveness, 

 tetrabromfluorescein, diiodofluorescein, homocol, and 

 ethyl red. The plates will keep in g(M)d condition for 

 two or three weeks. This will doubtless be a great con- 

 venience to many workers, and they will have the ad- 

 vantage of knowing exactly what sensitiser has been 

 employed. On the other hand there are advantag-es 

 sometimes in the use of n;ixed sensitisers as exempliiled 

 in commercial plates, and the experimental work that 

 guides manufacturers is not generally available. 



lyie Choice of Sensitisers -for Tied Light. — There ha\-e 

 lately been introduced several sensitisers for red in 

 addition to the two or three that have been known for 

 many years. Mr. \V. A. Scoble has recently com- 

 municated to the Royal Photographic Society the results 

 of his comparative experiments with all that are prac- 

 tically available, nine dyes in all. For getting sensi- 

 tiveness to the extreme red ])inacyanol is effective as far 

 as A, dicyanine nearly as far, while alizarine blue S 

 sensitises into the infra red, bnl h.is the disadvantage 

 of being, as others have found it, uncertain. In con- 

 sequence of this dilliculty be has selected pinacyanol 

 as the best for his work, though his observations on 



the colour changes of alizarine blue S when in solution 

 seem to go far towards eliminating the uncertainty of 

 its effects.* A noteworthy statement in his comnnniica- 

 tion is that he found no appreciable difference whether 

 he exposed the plates wet soon after being bathed, or 

 after being dried. The use of the undried plates saves 

 not only the time and trouble of drying, but the risk of 

 deterioration, which is considerable with some sensi- 

 tisers. 



Will Specially Coloured Sensitised Plates be much ap- 

 preciated? — It cannot but occur to tho.se who take an 

 interest in the matter to ask themselves whether the 

 facilities referred to in the two previous sections will 

 meet with due appreciation. If I had to answer this 

 question I should reply in the negative. Plates sensi- 

 tised for green have been on the market for about twenty 

 years, their advantages have been incessantly pro- 

 claimed, but even to-day they are very rarely properly 

 used, and it is often necessary to dra\v attention to their 

 (■xistoni-e. I should not be surprised if ten years hence 

 red sensitised plates will still stand in need of special 

 pleading so far as general photography is concerned. 

 It is the needs of the trade in the practise of three- 

 colour work that has been the chief incentive in the 

 investigations connected with red sensitisers. The 

 \ast majority of photographers probably do not know 

 that when they photograph a dark slated roof with a 

 ridge of light red tiles on it, the tiles come out darker 

 than the slates even when an " orthochromatic " plate is 

 used w ith a screen. The want of discrimination on the 

 part (jf photographers and those who look at their pro- 

 ductions is the cause of the absence of a demand for 

 correct representations, and no demand, of course, 

 means no supply. Improvements in red sensitiveness 

 will be quickly utilised by spectro.scopists because in 

 theii' work nothing can take its place; in microscopy 

 and certain other scientific work they will slowly find 

 appreciation; but for general purposes I fear that they 

 will remain neglected except by an enthusiastic few. 

 It must be admitted that it needs enthusiasm to make 

 the sacrifices that must be made if the possibilities of 

 suih plates are to be fully taken advantage of. And 

 certainly there is a great deal to be said in favour of 

 the facilities that at present are incompatible with red 

 sensitiveness. 



Dark Rooms. — Photographers when working away 

 from home, whether in the British Isles, on the Con- 

 tinent, or even in the more remote parts of the world, 

 will find " .V Directory of Public Dark Rooms," pub- 

 lished by Messrs. Dawbarn and W'ard at 3d., of much 

 use. It contains nearly three thousand entries. But 

 if anvone should happen to go on tour without such 

 information, he should look in likely places for a bold 

 dark blue Maltese cross six inches square on a rather 

 larger card or enamelled iron sheet. Where\er this 

 sign is displayed there is a dark room available, and 

 also a copy of the directory of dark rooms that may be 

 consulted by visitors. This arrangement is fairly well 

 established, for it has been in force some four or five 

 years, and in order to perfect it, the publishers would 

 bi- grateful to receive any intimations of errors or 

 omissions that are noticed in the directory. 



Notice. — The Royal Photographic Society has 

 arranged an exhibition of works by the members of the 

 Birmingham Photographic Society, at 66, Russell 

 Square, which will remain open to the public daily till 

 the i6th inst. 



• Since writing this Mr. Scoble has personally assured me that 

 by oljserving the precautions as to colour changes given in his 

 paper, he finds that the uncertainties hitherto found in the use of 

 alizarine blue are entirely eliminated 



