563 



MIRAGE. 



tance from the real object increases, because mirage is a reflection of the sides of the ob- 



tliey are what is known in the science of as- jects that are toward the observer, therefore 



troriomy as penumbral shadows. The sun casts they are presented in all their variety of color, 



two shadow s of all things that are smaller than and it is what the Scotch call looming. It 



FIG. 5. TOP-HAMPERS SINKING. 



itself. A man, for example, stands upon the 

 top of a mountain, as represented in Fig. 6. 

 One of his shadows tapers to a point, and is 

 the darkest because no light enters it. This is 

 called the umbral shadow ; the other grows 

 wider as distance increases, but is less dark, 

 because it is partially illuminated by the sun- 

 light. This is called the "penumbral shadow," 

 and to it are due the giants of Ben Lomond, 

 the gigantic specter of the Brocken, ships in 

 the air, etc. 



The looming of coast-lines differs a little in 



may be practically reproduced by laying a mir- 

 ror down horizontally, and lowering the e}e 

 to near its plane, when low objects on the op- 

 posite side will be seen, and if a semi-transpar- 

 ent screen be interposed the objects will appear 

 to be upon it, above the mirror, in the air, 

 while the double shadow of an object may be 

 produced, right and wrong side up, by simply 

 setting a toy ship down upon the mirror, and 

 holding a light so that the shadow of the toy 

 will be cast upon its surface, then the true 

 phantom-ship, erect and inverted, will be seen 



FIG. 6. UMBRA AND PENUMBRA. 



the causes of its production. The water is a 

 horizontal mirror lying at the foot of the line 

 of objects that it reflects at a low angle above 

 the horizon, and an eye on the opposite side of 

 the mirror will see those forms, provided there 

 intervenes a screen of vapor of sufficient den- 

 sity to intercept and transmit the forms, just 

 as a semi-transparent screen will transmit the 

 images of the magic-lantern. This form of the 



upon the opposite wall ; or. better still, n screen 

 of gauze, for then it will be more phantom- 

 like. 



The mirage of the desert is nothing more than 

 the looming of the bright sky near the horizon, 

 reflected by the sandy plains, which gives it 

 the appearance of water at some distance from 

 the observer, and any object rising above the 

 plain necessarily presents its dark side, which 



