Hutehins — Absorption and Thickness of thiol Films. 275 



The films are transparent to long leaves. 



The transmission of the films for very long waves was tested 

 as follows : A radiomicrometer was set up. Before the open- 

 ing of this was placed a thin blackened copper tank containing 

 water at room temperature. This tank could be raised and 

 lowered in a frame by pulling and releasing a string. Behind 

 the movable tank was a large cubical tank containing ice. The 

 radiation was therefore from a body at about 22° to one at 0°, 

 consequently the radiation maximum lies far out in the infra- 

 red at wave-length 10 /x. Exposures were made by raising the 

 movable tank, as noted above, and the ratio of the mean radio- 

 micrometer deflection with the film interposed to the mean 

 without the film gives the transmission. 



The following is a typical observation : 



Temp, of movable tank 22°-2 



" of stationary tank 0°-0 



Mean deflection with film 138-9±0"23 



" " without film 139-6±ci-21 



Transmission 0-995 



Several films have been tested in like manner, all showing an 

 absorption of less than one per cent ; whereas a plate of rock 

 salt o - 4 cm thick, newly and perfectly polished by Brashear's 

 method, gave an absorption of 22 per cent under the same con- 

 ditions. It would appear, therefore, that a nearly perfect win- 

 dow for radiometric instruments has been found, provided only 

 that the instrument is not to be exhausted. 



A thin coating of collodion may be of use in protecting the 

 surfaces of salt prisms and lenses from moisture. I polished 

 two plates of salt and coated one by flowing it with very dilute 

 collodion. The two have been exposed side by side for two 

 weeks. The coated plate is still perfect, whereas the uncoated 

 clouded in a day or two. It is too early, however, to know if 

 the protection is permanent. A thin collodion film does 

 furnish a permanent protection of silvered mirrors against 

 tarnish. A silvered mirror in this laboratory turns yellow in 

 a week owing to sulphur gases from a nearby railroad, but 

 when flowed with a very dilute collodion it remains untarnished. 

 I have such a mirror that has been exposed for three years 

 that is as perfect as when new. The films are so thin that 

 they in no wise impair the optical qualities of the surfaces to 

 which they are applied. 



Thick/tess of the films. 



It is of interest to know the thickness of these black films. 

 One film where it is doubled back upon itself over the wire 

 ring used as a support, thus giving a double thickness, showed 



