754 On some Iridescent Films. 



in this acidified water showed an advantage. A special ex- 

 periment confirmed the idea. Two plates, coated similarly 

 with silicate and dried a few hours before, were immersed, 

 one in ordinary tap water, the other in the same water 

 moderately acidified with nitric acid. After some 24 hours 

 soaking the first film washed off easily, but the second had 

 much greater fixity. There was now no difficulty in pre- 

 paring films capable of showing as good colours as those of 

 the old plates. The best procedure seems to be to dry off the 

 plates before a fire after coating with recently-filtered silicate 

 solution. In order to obtain the most suitable thickness, it 

 is necessary to accommodate the rapidity of drying to the 

 strength of the solution. If heat is not employed the strength 

 of the above given solution may be doubled. When dry the 

 plates may be immersed for some hours in (much) diluted 

 nitric acid. They are then fit for optical examination, but 

 are best not rubbed at this stage. If the colours are suitable 

 the plates may now be washed and allowed to dry. The full 

 development of the colour effects requires that the back of 

 the plates be treated. In my experience grinding gives the 

 best results when the lighting is favourable, but an opaque 

 varnish may also be used with good effect. The comparative 

 failure of such a treatment of the old plates was due to the 

 existence of films upon both sides. A sufficiently opaque 

 glass, e.g. stained with cobalt or copper, may also be em- 

 ployed. After the films have stood some time subsequently 

 to the treatment with acid, they may be rubbed vigorously 

 with a cloth even while wet ; but one or two, which probably 

 had been rubbed prematurely, showed scratches. 



The surfaces of the new films are not quite as glassy as the 

 best of the old ones, nor so inconspicuous in the air, but there 

 is, I suppose, no doubt that they are all composed of silica. 

 But I am puzzled to understand how the old plates were 

 manipulated. The films cover both sides without inter- 

 ruption, and are thinner at all the four corners than in the 

 interior. 



The extraordinary development of the colours in water as 

 compared with what can be seen in air led me to examine 

 in the same way other thin films deposited on glass. A thin 

 coat of albumen (without silicate) is inconspicuous in air. 

 As in photography it may be rendered insoluble by nitrate 

 of silver acidified with acetic acid, and then exhibits good 

 colours when examined under water with favourable illu- 

 mination. Filtered gelatine, with which a little bichromate 

 has been mixed beforehand, may also be employed. In this 



