EXPERIMENTS WITH INVISIBLE LIGHT—WOOD. 159 
practically all of these very short waves are scattered by the atmos- 
phere, and we have no shadows even in full sunlight. 
We will now run through the series of infra-red pictures as rapidly 
as possible, for I have a considerable number of them. The one 
which is on the screen is one of the finest in the collection (pl. 1). It 
was made in the park at Florence, and shows the long drive, over- 
shadowed by trees, the one in the foreground being particularly fine 
in appearance. The next one (pl. 2) was made at the bottom of 
one of the old quarries or latomisz at Syracuse, the view looking out 
through a cavelike 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 brighter 
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 that the surface of the earth, as seen from a neigh- 
boring planet, would be seen through a luminous haze, equal in 
brilliance to the blue sky on a clear day; that is, it would present 
much the same appearance as is presented by the moon when seen at 
noonday. 
_ 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 can not employ glass lenses. Quartz, on the 
other hand, is exceedingly transparent to these invisible 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 fulfills 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, rich 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 
