184 Professor K. W. Wood [May 19, 



in appearance. The next one (Fig. 2) was made at the bottom of 

 one of the old quarries or latomise at Syracuse, the view looking out 

 through a cave-like formation at a group of almond trees, with which 

 the quarry is overgrown. 



Here is a fine row of cypresses growing by an old gate, taken on 

 a somewhat hazy day, with the sky appearing a little lighter than 

 usual. Some of the pictures show the advantage gained in bringing 

 out the detail of distant objects seen through the atmospheric haze, 

 and it does not seem impossible that photographs of the Iwighter 

 planets made through an infra-red screen might prove interesting if 

 the planets are surrounded by a light scattering atmosphere, for we 

 must bear in mind chat the surface of the earth, as seen from a neigh- 

 bouring planet, would be seen through a luminous haze, equal in 

 brilliance to the bhie sky on a clear day — that is, it would present 

 much the same appearance as is presented by the moon when seen at 

 noon-day. 



We will now look into the question of how things would appear if 

 our eyes were sensitive only to ultra-violet light. In applying the 

 same method which we have used for the infra-red, we require a 

 screen which is opaque to all visible light, but which transmits the 

 ultra-violet. 



Glass is opaque to these rays, cutting them off almost completely, 

 and for this reason we cannot employ glass lenses. Quartz, on the 

 other hand, is exceedingly transparent to these invisilDle rays, but it 

 is a little difficult to find a medium which is transparent to them and 

 at the same time quite opaque to visible light. Indeed, there is only 

 one substance known which completely fulfils such a condition, 

 namely, metallic silver. If we deposit chemically a thin film of 

 metallic silver on the surface of a quartz lens, a certain amount of 

 ultra-violet radiation between 3000 and 3200 is able to struggle 

 through and form an image on the plate. 



I have used silver films through which the filament of a tungsten 

 lamp is invisible. The best thickness is that at which the tungsten 

 lamp is just barely discernible. If the objects to be photographed 

 are illuminated with the light of an electric spark, or some other 

 source ricli in ultra-violet rays, much thinner films of silver can be 

 employed, but in the case of sunlight, which has passed through the 

 earth's atmosphere, the ultra-violet in the region for which silver has 

 its lowest reflecting power and greatest transparency has been so 

 tremendously weakened by atmospheric absorption, that it is necessary 

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

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

 rays, which ai'e capable of traversing glass. 



As an illustration of the behaviour of silver films of different 

 thicknesses, 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. 



