168 On the Mirage of India. [No. 2. 



meet the eye from one ripple, another from another ripple, with no 

 appreciable difference, and the aggregate will suffice to paint an image 

 upon the retina, see PL VIII. fig. 3. 



There are other effects of vapour less known than those above 

 described. One of these, is to magnify greatly any object seen through 

 the medium. This may sometimes be affected by scattering and 

 disordering the image : but is, I think, more generally a mere illusion 

 occasioned by exhibiting the figure with a faintness of outline as if 

 seen at remote distance, without any diminution of apparent bulk. 

 Thus, in crossing the desert on my approach to the Bolaun pass, 

 I saw by moonlight a camel magnified to gigantic dimensions : an 

 effect, which I am inclined to attribute to the figure being dimmed 

 by mist, so as to appear remote, when it was really close to the eye 

 and subtending of course a considerable angle. The soft fall of the 

 camel's foot upon sand creates no sound and adds greatly to the effect. 



An illusion of the same character I have elsewhere seen beautifully 

 exhibited, viz. : upon the highest summit of the Simla mountain. 

 There, as I have sat gazing upon the glorious landscape, it has been 

 gradually removed to immeasurable distance by a transparent and 

 imperceptible vapour, which crept up from the valley over the mountain 

 brow ; and which as it gradually rolled past, as gradually brought back 

 objects to their original proximity, with an effect truly magical. 



That species of mirage so often described by travellers of the 

 desert I have not mentioned, because I have not met with it under 

 circumstances favourable to an examination of the phenomenon. I 

 allude to the appearance of water in spots utterly dry, an illusion to 

 which even the most experienced are at times liable ; so perfect is the 

 resemblance. This mirage appears to be an isolated stratum of almost 

 transparent but dense vapour occupying accidental hollows, depressed 

 beneath the observer's eye. It is commonest at night in India. The 

 vapour thus accumulated having a higher refractive power than the 

 atmosphere, not only has the gleam common to water, but reflects 

 images of objects beyond it, precisely after the fashion of standing 

 pools. It appears to be commonest in saline deserts, where the extreme 

 heat evolving particles of salt in solution with the vapour, forms a 

 vapoury stratum of greater density than that arising from pure water, 

 and of course of higher refractive power. The effect exhibited in 



