158 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
If the object to be photographed gives off visible rays in addition 
to the invisible ones, it is necessary to remove these by a suitable 
screen or ray filter. We will begin by considering some remarkable 
effects which are obtained when sunlit landscapes are photographed 
by means of the obscure rays at the extreme red end of the spectrum. 
A screen can be prepared which transmits these rays, and is at the 
same time opaque to all other radiations, by combining a sheet of the 
densest blue cobalt glass with a solution of bichromate of potash or 
some suitable orange dye. 
Such a screen transmits a region of the spectrum comprised 
between wave lengths 6900 and 7500. Though this region is visible 
to the eye if all other rays are cut off, it is so feeble in its action that 
it plays no part in ordinary vision, being overpowered by the other 
radiations. We may thence, for convenience, call photographs made 
through such a screen infra-red pictures, though the infra-red region 
is usually considered as beginning at the point where all action upon 
the human retina ceases. 
The photographs which I am now going to show you were taken 
through such a screen, with the spectrum plates made by Wratten and 
Wainwright. The time of exposure was about three minutes in full 
sunlight, with the lens stop set at f/8. The views were, for the most 
part, made in Sicily and Italy, and have a very curious appearance, 
for while the sky comes out in all of them almost as black as mid- 
night, the foliage of the trees and the grass come out snow white. 
This peculiar effect results from the failure of the atmosphere to 
scatter these long rays. The green leaves, however, reflect them very 
powerfully, or, more correctly, transmit them, since we are dealing 
with pigment or transmission color. If we look at a landscape 
through the screen, carefully protecting the eye from all extraneous 
light with a black cloth, we shall find that the trees shine with a 
beautiful rich red light against a black sky. This condition obtains 
only on very clear days, for the presence of the least haze in the air 
enables it to scatter the long rays, and you will notice that in those 
pictures which show the sky down to the horizon there is a pro- 
gressive increase in its luminosity as we pass from the zenith down- 
ward,.as a result of the greater thickness of the mass of air sending 
the scattered rays to the camera. 
Another point to be noticed is the intense blackness of the shadows 
in the infra-red pictures, due to the fact that most of the ight comes 
directly from the sun and little or none from the sky, which reminds 
one forcibly of the conditions which obtain on the moon, where 
there is no atmosphere at all to form a luminous sky. 
When we come to the subject of photographs made with ultra- 
violet light, we shall find that we have the conditions reversed, for 
