164 On tie Mirage of 'India. [No. 2. 



entirely disappeared, and I questioned whether I had not been suf- 

 fering some illusion of the eye or mind : for I was not then aware 

 that Mirage is known in India. A residence in Malwa, where it is 

 common, made me familiar with some of its phases, and as I have 

 never met with an intelligible description of the process of this illusion, 

 a slight sketch may be acceptable to the general reader. 



The Mirage most commonly observed in India is the effect produced 

 upon distant objects, by means of a mirror, suspended with its surface 

 downwards at the distance of from 60 to 250 feet from the earth, half 

 way between the object and the eye of the spectator. This mirror is 

 a stratum of dense but transparent and scarcely visible vapor, evolved 

 from the dewy earth by the action of the sun's rays, generally about an 

 hour or two hours after sunrise. The refractive power of this vapor 

 being greater than that of the atmosphere, acts precisely as would a 

 mirror of glass similarly suspended : that is, it catches the reflection 

 of distant objects and exhibits them hanging in reverse. But, be- 

 ing slightly agitated by the air and by the action of the sun upon 

 its upper surface, it slightly confuses every outline ; giving a wavy 

 appearance, as we see in images reflected by a running stream. And 

 as the reflected image is seen in juxtaposition with the substance : 

 and as the stratum of vapor is connected with the earth, by less dense 

 currents rising up to join it, it follows that the lower portion of the 

 reflection is prolonged downwards until it meets the summit of the 

 substance. The substance and its reflection are thus blended together 

 at their respective summits : a respect in which Mirage differs from 

 the reflections in a clear lake. The object and its reflection in the 

 latter meeting together at their bases respectively. 



I have described the stratum of reflecting vapor as hanging mid- 

 way between the object and the spectator ; because this its position is 

 essential to the production of Mirage. But generally the vapor hangs 

 in one continued canopy from the object to the eye of the spectator. 



This reflecting canopy exhibits the images of distant objects alone, 

 because its substance is not sufficiently dense to repel those rays of 

 light which fall upon it at any sensible angle of incidence. It is only 

 when the angle of incidence is extremely small, that the ray will 

 rebound from the surface of the vapor. It follows that supposing 

 the strength of illumination sufficient, the image will be distinct in 

 proportion to the distance of the object. 



