VARIOUS SOLUTIONS. 49 



asphaltum may be applied to the rock-salt as a thin varnish, the volatile 

 parts of which evaporate in a short time. 



Asphaltum varnish, applied in an extremely thin coating, forms an excel- 

 lent protecting surface for radiometer or radiomicrometer windows made 

 of rock-salt. It might also be used advantageously as a covering for 

 rock-salt prisms to protect them from moisture. The pair of films of 

 asphaltum to be used for such purposes can be made extremely thin, and 

 are almost transparent, as shown in fig. 31, curve d, which was of a light 

 amber color showing but little absorption in the visible spectrum. The 

 film was prepared from commercial asphaltum varnish, which is a very 

 thick solution, by diluting it with gasoline. This precipitates the very 

 insoluble constituents, which apparently are simply held in suspension, 

 and by experimenting on the amount of gasoline to be added, a film or 

 pair of films of any desired thickness may be produced by a single dipping 

 of the plate or prism. Of course, a thicker film may be formed by using 

 a thicker solution, or by applying several coats of the thin solution. By 

 filtering the dilute solution through cotton wool or through filter paper 

 the black insoluble asphaltum particles are removed. The dried film is 

 then homogeneous and perfectly free from even traces of black particles. 

 The gasoline must be quite free from moisture, otherwise there is likely to 

 be a slight action on the rock-salt surfaces, and the dried film will not be 

 so homogeneous as that obtainable on a glass plate. It may be possible 

 to apply a homogeneous film of paraffin or of ozokerite, which is white, 

 and which, on account of its low refractive index, will reflect less than a 

 rock-salt surface; but neither of these substances are as satisfactory as 

 asphaltum purified, as indicated above. Such surface coverings for hydro- 

 scopic substances are, of course, useful only where the radiometer and 

 prism are used in making relative comparisons, i. e., it is not adapted to 

 making spectral energy measurements of sources of radiation, where the 

 absorption of the apparatus and the atmosphere must be eliminated. 1 

 Curve d is for two films on a plate of glass, 0.175 mm - thick. The values 

 are obtained by dividing the observed transmission, when the glass was 

 coated with asphaltum, by the observed transmission through the clear 

 glass. The fact that the transmission is greater than 100 per cent is due 

 to the higher reflecting power of glass, for which no correction was made. 



1 Since writing this, Dr. A. Trowbridge, at the Washington Meeting of the American 

 Physical Society, April, 1908. described the transmission of extremely thin films of collodium 

 (Carnegie Publication No. 65, p. 60, fig. 46) and their application to rock-salt as a protecting 

 surface. For the region of the spectrum up to 6 m there are no prominent absorption bands, 

 and the small ones at 3.3 p. have almost entirely disappeared in the thinnest films, which are 

 of the order of a light-wave in thickness. Collodium is said to be slightly hydroscopic, but 

 whether it is sufficiently so to become separated from the rock-salt is unknown. At this 

 meeting it was learned that at the Astrophysical Observatory it is proposed to use asphaltum 

 varnish to protect the large rock-salt prism which is about 18 cm. high. 



