MIRAGES—GORDON 329 
carried away by a shallow stream perhaps 8 feet wide. Some little dis- 
tance below the last inflow there is, or was, a small footbridge spanning 
the brook and offering an excellent view of its gravelly bed. This was 
made up of parallel strips of color—reds, browns, greens, yellows, 
blues, and so on—not much more than an inch wide with lines between 
them sharply drawn. Each stripe represented the streambed carrying 
the flow from some particular geyser. Algae, differing in color, had 
adapted themselves to the temperature and mineral content of the 
various basins, and the overflow carried some of them with it to find 
a new home on the pebbles and gravel of the streambed, each one mark- 
ing clearly the flow from its own basin. These little streamlets had 
flowed along for some distance without mixing, and so continued as 
far as the eye could follow. They presented visible evidence, thanks 
to the algae, of the ability of a stream of water, probably with slight 
difference in density from its neighbors, to keep its separate identity 
while flowing along in contact with other streams. If it had not been 
for the algae, this phenomenon would have been as invisible to us as 
most of the air lenses in the general airmass. 
Getting back to these lenses, let us look at the diagrams (fig. 1) which 
attempt to make clear how or why a bending in the light path occurs; 
then, by making use of the air lens in the two positions, dense side up 
and less dense side up, how the effects are brought about. Results from 
the two positions of the lens differ so radically that it seems best to 
designate them as lenses No. 1 and No. 2. The function of No. 1, 
denser at the top than at the bottom, is almost exclusively that of mak- 
ing water seem to be where there is none. As the diagram shows, lens 
No. 1 will make an object seem to be lower than it actually is. The 
object is usually a bit of blue sky near the horizon which is made to 
appear on a road or on a dry lake bed, where it looks like water. This 
lens will be formed almost exclusively in contact with sun-heated 
ground or roadway, but it has also been observed against walls heated 
by the sun. It has been formed in the laboratory as well. 
The No. 2 lens, denser at the bottom than at the top, has the reverse 
effect of No. 1 in that it makes an object, or part of it, seem to be higher 
than it is. The activities of No. 2 lens produce three types of effects: 
(1) The lens when lying on a ridge between object and observer, almost 
invariably produces a good, clear picture of some object, normally 
hidden behind the ridge, that is lifted into sight by the mirage. (2) 
When the lens is in the temperature inversion layer of the atmosphere 
from a few feet to thousands of feet above the ground, it can distort the 
appearance of an object, in addition to raising it up. (3) Also, when 
the lens is in the temperature inversion layer, it can pick up and 
transport a picture or image and show it a few miles or hundreds of 
miles away. The picture may be erect or inverted, and sometimes 
