k 



TRANSACTIONS OF SECTION B, 661 



In India, in 1875, at my suggestion Colonel Waterhouse used bromide of silver 

 collodion plates dyed with eosin. The success of his experiments demonstrates 

 that eosin was the best optical sensitiser for collodion plates. Later Ducos de 

 Hauron employed orthochromatic plates in his photochrome or three-colour 

 printing process. Attout-Tailfer next introduced isochromatic gelatine dry 

 plates dyed with eosin or its derivatives, in conjunction with an alkali. I used 

 azaline — a mi.xture of quinoline blue and quinoline red— for the same purpose ; 

 whilst Eder, of Vienna, recommended erythrosin (tetra-iodo-tluorescein) as the 

 best optical sensitiser for dry plates. In 1885 Obernetter and I showed that by 

 the use of eoside of silver pla^s it was possible to dispense with a yellow screen 

 for ordinary landscape photography. I find that the best results are obtained with 

 a film containing 1,000 parts of collodion (containing seventeen of cotton) to 

 three parts aurantia. The exposure required in this case is four times as long 

 as when no yellow screen is employed. For landscapes only two parts of aurantia 

 are necessary, and the exposure required is only two and a half times longer than 

 without a screen. Another important factor in regard to orthochromatic plates is 

 the action of the developer. My own results show that the impressions of the 

 blue rays develop before those of the red and yellow, and, therefore, when using 

 eosin dyed plates they should be developed until all the yellow parts of the picture 

 are visible in the negative. It is usually stated that the relative value of colours 

 in a landscape approximates more closely at sunrise and at sunset than at noon, 

 and, therefore, that in the former case a yellow screen is unnecessary when using 

 orthochromatic plates. This would be true were direct sunlight the only agent, 

 but since difl'used light also comes into play it is not the case, for Crova has shown 

 that the proportion of blue rays in diffused light increases as the sun goes down, 

 whilst the reverse bolds for direct sunlight. With orthochromatic plates the 

 results depend on (1) the colour sensitiveness of the plate; (2) the proportion 

 of the different coloured rays in the diffused light : and this proportion, I find, varies 

 from day to day. 



I gather from Captain Abney's paper in the 'Photographic Journal ' that the 

 sensitiveness of the plates he employed for yellow rays was only fths of that 

 for blue. To this fact I attribute the unsatisfactory results he obtained. In 

 Germany we use eosin dj'ed plates, the sensitiveness of which is five times 

 greater for yellow rays than for blue rays. Such plates can be used for landscapes 

 without a yellow screen. The prints exhibited to the Section show the com- 

 parative results obtained with ordinary plates and plates of this kind. The 

 pictures were taken at 3 p.m. 



2. On the Sensitising Action of Dyes on Gelatino-br amide Plates. 



By C. H. BOTHAMLEY. 



Although large numbers of dyes have been examined since Dr. H. W. Vogel's 

 discovery in 1873, very few exert any marked effect in making gelatino-bromide 

 plates sensitive to the less refrangible rays of the spectrum. Only cyanin and the 

 dyes of the eosin group (including the rhodamines), with perhaps malachite green, 

 chrysoidine, and alizarin blue, can be said to exert any useful effect. 



The main points established by previous observers may be summarised as follows : 

 (1) The dyes that act as sensitisers are readily affected by light when they are in con- 

 tact with fabrics, paper, &c. ; (2) in order that a dye may act as a sensitiser it must 

 have the power of entering into intimate union with silver bromide, forming a 

 kind of ' lake ' ; (3) and it must show a strong absorption band for the particular 

 rays for which it is to sensitise. Although these statements hold good for all the 

 dyes that are known to act as sensitisers, it is important to observe that the con- 

 verse is not necessarily true. Several dyes having all these properties show no 

 appreciable sensitising action. 



Experiments by Dr. E. Vogel on the rate of fading and the sensitising action of 

 the eosin dyes, led him to the conclusion that the order of sensitising effect coin- 

 cides with the order of fading when the dyes are exposed to light. The order in 

 which Vogel places the dyes does not, however, correspond with the order of fading 



