

Note on a new Form of light Plane Mirrors. 441 



the film, left after the evaporation of the solvent, on a ring previously 

 placed beneath the surface. 



Good films can also be obtained in this way from copal varnish, but 

 they are more difficult to handle, and the intrinsic strength of the 

 material is not nearly as great as that of pyroxyline. 



In order to lift the films off the surface of the water without folds, 

 and without getting any of the parts doubled, it is desirable that the 

 ring used for the purpose should be not less than half an inch deep, i.e. t 

 a length of half an inch cut off a tube of the required diameter, or a 

 strip half an inch wide bent into a circle and the ends joined. If the 

 depth is less than this the overlapping part of the film is liable to be 

 carried round the lower edge by the capillary action of the water, and 

 to spread itself irregularly on the under surface of the stretched part 

 while being lifted. 



The ring should be rested on a wire tripod with long turned-in feet 

 so that the pyroxiline solution may spread well beyond its edges before 

 encountering the tripod legs ; and, in order that the film while being 

 lifted may not be broken by the drag of the water enclosed by it and 

 the ring, air should be introduced inside the latter through a pipette 

 during the process. A little practice makes it quite easy to secure uru% 

 form films with certainty in this way. 



After removal from the water the film should be allowed to drain 

 for some minutes until the surface is free from drops, and it may then 

 be stretched over the worked ring whose edge is to form the boundary 

 of the mirror. In order to do this the worked ring is placed on a 

 circular support some inches in height and a little larger in diameter 

 than the ring itself. The film, which at this stage of drying is tough 

 and extensible, is then laid in position over it, and the lifting ring 

 forced downwards until the film tears away. It will be then found that 

 the film is left well stretched over the worked ring and adherent to its- 

 edge and side. 



The lifting rings should be about half an inch larger in diameter 

 than the worked rings. 



If the film is not much more than a twenty-thousandth of an inch 

 in thickness its surface is glassy and reflects light well, but if much 

 thicker than this .it tends to show a crape-like or finely grained struc^ 

 ture, and its quality as a reflector is not so good. 



I found at first great difficulty in silvering the films : in fact as the 

 film comes from the water, it refuses altogether to receive the silver 

 deposit, but if well washed under a very gentle stream of distilled 

 water as soon as it is stretched on the mirror ring, and then thoroughly 

 dried, a fair coating of silver may be obtained. 



It seems essential that the amyl acetate should be completely evapor- 

 ated, and this being accomplished, the other requisite is absolute 

 surface cleanliness both of the film and the silvering bath. 



