330 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
~ 
—— = (FO 
= % E> = 3 
ine 2S ee =o os 
=> ~ ae aS 
. —_— 
DECREASING 
‘ — i ——® 
Density With es 2 aren 
INCREASING Paap od eRe 
TEMPERATURE AIR LENS 
= ae — 
— sos" LENS No.2 — Usuy Fr 
— e110" ma => ~2™ Og, 
=a a pire “cr - 
—_peravt ed 
Ficure 1.—Mirage formation depends on a lens of air similar in function to lenses in our 
cameras and telescopes. Density of the air forming it must increase or decrease progres- 
sively from top to bottom. Speed of light increases as it moves into less dense air, and 
conversely. In the first diagram, the left side of a shaft of light enters the lens first, 
picking up speed. At all times, while within the lense, it 1s in thinner air and moving 
faster than the right side, covering the longer are while the right side covers the shorter 
one. ‘The result is a change in direction of the light path. As drawn the center diagram 
would be known as lens No. 1 with the quality of making the object of the mirage seem 
lower than it is. ‘Turned over, as in No. 2, it works equally well in making the object 
seem higher than it is in reality. No. 1 nearly always rests on the ground. No. 2 may be 
on a ridge, between object and observer, or aloft in the free air. ‘To facilitate drawing, 
change in direction as shown by diagrams is made much greater than we find in nature, 
as is the depth of the lens in proportion to diameter. Because the rate of density change 
is rarely uniform, the path of direction-change arc is unlikely to be a simple curve. Its 
penetration is likely to be at, or near, the bottom of lens. 
both positions are shown. Actually, of course, mirage making is often 
not nearly so simple as might be inferred from the diagrams. 
Some types of mirage, such as the seeming strip of water in the road, 
or water in a dry lake bed, may be seen very frequently. Others ap- 
pear often during certain seasons of the year. Wind, humidity, cloudi- 
ness, or storm can effectively interfere with their habits. Then there 
are those that appear only several times a year or once in a number of 
years, or even once in a lifetime or within the memory of man. (One 
of my best mirage stories is nearly a hundred years old and is ap- 
parently the only such appearance over that little town in all that 
time.) They are seemingly phenomena of habit, showing in the same 
place over and over again and, just as stubbornly, not showing in other 
places. Mirages do not broadcast their pictures as does a moving- 
picture screen; they may be seen only from a limited area, established 
by the lens’s angle of refraction. In the case of the No. 1 lens, the field 
of observation is sometimes a broad-rimmed circle completely sur- 
rounding the mirage. Actually as we move around that circle, the bit 
of sky pictured is constantly changing, but it looks the same. 
