160 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1911. 



its lowest reflecting power and greatest transparency has been so 

 tremendously weakened by atmospheric absorption, that it is neces- 

 sary to employ thick films and long exposures, otherwise the action 

 upon the photographic plate results chiefly from the violet and ultra- 

 violet rays, which are capable of traversing glass. 



As an illustration of the behavior of silver films of different thick- 

 nesses, used as ray filters, we may take some pictures which were 

 made for the purpose of studying the reflecting power of various 

 metals, suitable for telescope mirrors, for ultra-violet photography. 

 As silver reflects only about 4 per cent of the ultra-violet in the 

 spectrum range for which it is transparent, a silvered glass reflecting 

 telescope for this purpose is obviously out of the question. Speculum 

 metal is fairly suitable, but speculum mirrors of large size are trouble- 

 some, and difficult to procure. I accordingly worked out a method 

 of depositing nickel on glass. The glass is first silvered, and then 

 electro-plated with nickel, by a process which I have described 

 recently in the Astrophysical Journal (Dec, 1911). The double 

 sulphate of nickel and ammonia is used with one or two dry cells. 

 The solution must be very dilute (10 grams or less to the liter), 

 otherwise the nickel strips the silver from the glass. We have here 

 four pictures of a silvered glass dish, partially plated with nickel 

 (pi. 3, fig. 1). The silvered portion is marked Ag, the nickel Ni, 

 while at G we have a spot of clear glass from which the metal has 

 been removed. The dish stands against a flat plate of polished 

 speculum metal Sp, and the metal surfaces reflect the light of the 

 sky to the camera. The first picture was made by blue and violet 

 light without any ray filter, and as you see the glass surface G is 

 quite black, while the silver reflects much more powerfully than the 

 nickel. The following three pictures were made with a quartz lens, 

 coated with silver films of increasing thickness. The silver and 

 nickel reflect to about the same degree in the second picture, in the 

 third the silver is much darker than the nickel, while in the fourth 

 the silver is seen to reflect no more than the spot of clear glass G. 

 This last was made through a film, through which a tungsten lamp 

 was invisible. If these ultra-violet rays were visible to us, metallic 

 silver would appear to have about the same reflecting power and 

 appearance as anthracite coal. 



We will next take up the action of our atmosphere on these ultra- 

 violet rays. I have taken two photographs of a man standing in the 

 road in full sunshine, in the one case by ordinary light and in the 

 other by ultra-violet radiation. In the latter the shadow is com- 

 pletely absent. Ultra-violet behaves in exactly the opposite way to 

 the infra-red. The infra-red rays are enabled to drive through the 

 atmosphere without being scattered laterally by the molecules of the 

 air or the dust particles. The short or ultra-violet rays, on the other 



