observed at sunrise from the Summit of Mount JEtna. 221) 



What proof was there that the image was that of Mount 

 ./Etna ? — To say that there was no other mountain whose 

 image could be thus seen reflected, would be merely to enun- 

 ciate an opinion, and far from giving such an answer as the 

 question required. When an object is seen by reflection from 

 a concave mirror, it appears inverted as well as diminished ; 

 it is not said, whether, in this instance, the image was or was 

 not inverted. But, on the supposition that the appearances 

 were produced by unequal atmospheric refraction, the upright 

 or inverted position of the image would be contingent on the 

 particular state of the air at the time of observation. A dis- 

 tinct and perfect image of any part of the earth's surface ap- 

 pearing elevated above the horizon is never produced by sim- 

 ple reflection, but is the effect of unequal refraction ; hence 

 the image, as above described, could not be that of Mount 

 .<Etna, for the place occupied by the observer could not be re- 

 presented to him by any reflective power of the atmosphere ; 

 and it would be absurd to suppose the effect to be produced 

 by refraction. If it were allowable to suppose that what is 

 described as a perfect and distinct image of ./Etna, elevated 

 above the horizon, was merely a part of the shadow of that 

 mountain rendered visible by the presence of a white cloud 

 or misty vapour, somewhere between the place of the observer 

 and the horizon, the description, though inaccurate, would 

 be so far intelligible, and the image might disappear as the 

 shadow of the mountain decreased. The well-known opti- 

 cal phenomenon seen on the Hartz mountains, and vulgar- 

 ly named the Spectre of the Brocken, is of this nature : 

 When a person stands at sunrise on the top of the mountain, 

 and looks toward the west, if there be any white clouds, or 

 even a thin misty vapour, in that direction, he sees his own 

 shadow on these clouds immensely magnified, but so well de- 

 fined as to render his form, attitudes, and motions, distinctly 

 visible. If, by observing the representation of smoke issuing 

 from the crater, or by other means, Mr Hughes had determin- 

 ed, with certainty, that the image was that of Mount jEtna, 

 there is little reason to doubt that the above would have been 

 the true explanation ; but his description taken as a whole, 

 and particularly the circumstance of the image being diminish- 



