EXPLANATION OF PHENOMENA. 155 



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 obvi- 

 ously 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 two 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. When- 

 ever 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 reflexion. If an 

 object, therefore, were strongly illuminated, a 

 sufficiently distinct image, or rather shadow of it, 

 might be seen by reflexion from strata of air of 

 different density. As the temperature at which 

 moisture is deposited in the atmosphere varies 

 with the density of the air, then at the same 

 temperature 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 

 sufficiently 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. 



