Itm] on Recent Experintents with Invisible Light. 185 



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

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

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

 metal is fairly suit<ible, 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 litre), 

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

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

 (Fig. 3). 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 throucrh the 

 atmosphere without being scattered laterally by the molecules oi the 

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

 hand, are completely scattered, so that the greater part of the ultra- 

 violet light which reaches the surface of the earth comes from the sky 

 and not directly from the sun. If our eyes were sensitive only to 

 ultra-violet we should find the world appearing not greatly different 

 from the aspect which obtains at the time of light fog. We should, 

 indeed, see the sun, but it would be very dull, and there would l)e no 

 shadows, just as there are none on a foggy day. We should walk the 

 earth like Peter Schlemeil, the shadowless man of the German fable. 



The next picture (Fig. 4) illustrates the opacity of ordinary window- 

 glass to ultra-violet radiation. It will be noticed that there is no trace 

 of the landscape seen through the glass window, although it is clearly 

 rendered in the companion picture taken with visible light. Another 



