some Iridescent Films. 753 



great measure to the special localized character o£ the illumi- 

 nation. The substitution of strong brine for water made no 

 perceptible improvement. 



At this stage I found a difficulty in understanding fully 

 the behaviour of the unground plates. In some places the 

 black would occasionally be good, while in others it had a 

 washed-out appearance, a difference not easily accounted for. 

 A difficulty had already been experienced in deciding upon 

 which side of a plate the film was, and had been attributed 

 to the extreme thinness of the plates. But a suspicion now 

 arose that there were films upon both sides, and this was 

 soon confirmed. The best proof was afforded by grinding 

 away half the area upon one side of the plate and the other 

 half of the area upon the other side, Whichever face was 

 uppermost, the unground half witnessed the presence of a 

 film by brilliant colouration. 



Attempts to produce silicate films on new glass were for 

 some time an almost complete failure. I used the formula 

 given bv Abney (Instruction in Photograph v, 11th edition, 

 p. 342) :- 



Albumen 1 part. 



Water 20 parts, 



Silicate of Soda solution of syrupy consistency . 1 part. 



But whether the plates (coated upon one side) were allowed 

 to drain and dry in the cold, or were more quickly dried off 

 over a spirit flame or before a fire, the resulting films washed 

 away under the tap with the slightest friction or even with 

 no friction at all. Occasionally, however, more adherent 

 patches were observed, which could not so easily be cleaned 

 off. Although it did not seem probable that the photo- 

 graphic film proper played any part, I tried without success 

 a superposed coat of gelatme. In view of these failures I 

 could only suppose that the formation of a permanent film 

 was the work of time, and some chemical friends were of 

 the same opinion. Accordingly a number of plates were 

 prepared and set aside duly labelled. 



Examination at intervals proved that time acted but slowly. 

 After six months the films seemed more stable, but nothing- 

 was obtained comparable with the old iridescent plate*. It 

 is possible that the desired result might eventually be achieved 

 in this way, but the prospect of experimenting under such 

 conditions is not alluring. Luckily an accidental observation 

 came to my aid. In order to prevent the precipitation of 

 lime in the observing-dish a few drops of nitric acid were 

 sometimes added to the water, and I fancied that films tested 



