222 LETTERS ON NATURAL MAGIC. 



only have been produced by a duplication of one of the 

 figures produced by unequal refraction. 



The reflected spectre of Dr. Buchan standing upon 

 the cliff at Brighton arose from a cause to which we have 

 not yet adverted. It was obviously no shadow, for it is 

 certain, from the locality, that the rays of the sun fell 

 upon the face of the cliff and upon his person at an angle 

 of about 73 from the perpendicular, so as to illuminate 

 them strongly. Now there are t\vo ways in which such 

 an image may have been reflected, namely, either from 

 strata of air of variable density, or from a vertical stratum 

 of vapour, consisting of exceedingly minute globules of 

 water. Whenever light suffers refraction, either in 

 passing at once from, one medium into another, or from 

 one part of the same medium into another of different 

 density, a portion of it suffers reflection. If an object, 

 therefore, were strongly illuminated, a sufficiently distinct 

 image, or rather shadow of it, might be seen by reflection 

 from strata of air of different density. As the tem- 

 perature at which moisture is deposited in the atmosphere 

 varies with the density of the air, then at the same tem- 

 perature moisture might be depositing in a stratum of one 

 density, while no deposition is taking place in the adjacent 

 stratum of a different density. Hence there would exist as 

 it were in the air a vertical wall or stratum of minute 

 globules of water, from the surface of which a suffi- 

 ciently distinct image of a highly-illuminated object 

 might be reflected. That this is possible may be proved 

 by breathing upon glass. If the particles deposited upon 

 the glass are large, then no distinct reflection will take 

 place ; but if the particles be very small, we shall see a 

 distinct image formed by the surface of the aqueous film. 



The phenomena of the Fata Morgana have been too 

 imperfectly described to enable us to offer a satisfactory 

 explanation of them. The aerial images are obviously 



