24,8 t>r. Wollaston on double Images 



One morning, when the sun shone bright, I examined the 

 temperatures and refraction produced at the surface of a deal 

 bar painted green, about 8 feet long. 



A small thermometer in contact with the bar, lose to $6°; 

 at ^ of an inch distance, it stood at y<£. 



The refraction at the same time exceeded 20 minutes. 



To explain why red-hot iron occasions two images, while 

 solar heat produces but one, I imagine that the intense heat in 

 the former case rarefies the air for some small distance uni- 

 formly, and thereby affords the same -series of variations as 

 between other fluids of uniform density ; but that, in the latter, 

 the heat is conveyed off as fast as it is generated ; so that, as 

 there is no extent of medium uniformly rare, the densities cor- 

 responding to the concave portion rm, Fig. 5. of the curve 

 before described do not take place, but the phenomena occa- 

 sioned by the convex part md are alone produced. 



It must be remarked, that the vertical position of the surface 

 contributes greatly to increase the effect ; for, since the heated 

 air rises in the direction of the surface, its ascent has in this 

 case no tendency to blend it with the adjacent denser strata, and 

 hence very different degrees of density take place in the thick- 

 ness of £ of an inch ; so that, as the increments of density are 

 great, the refractions will be proportionally so ; but, where the 

 heated surface is horizontal, the ascent of the rarefied air into 

 the superincumbent denser strata renders the variations far more 

 gradual ; consequently, a heated surface of far greater extent 

 must be requisite, to produce equal refraction. 



However, over extensive plains, when the sun shines, some 

 degree of inversion is very frequently to be seen ; but the 

 inverted images are rarely well defined, unless over a very even 



