246 Dr. Wollaston on double Images 



when my eye was removed four or five feet from the vessel, the 

 effects were the same as in the preceding experiments with dif- 

 ferent fluids ; above any object viewed through the cold water, 

 I could distinguish two images of it, the one inverted, the other 

 erect, as usual; but these appearances did not continue more 

 than five or six minutes. 



Having thus established, by experiments sufficiently varied, 

 that the contiguity of two fluids of unequal density is capable 

 of occasioning all the appearances that have been observed, I 

 shall proceed to shew by what means the air may be made to 

 exhibit similar phaenomena. 



Exper. 5. I heated a common poker red-hot, and held it so 

 as to look along the side of it, at a paper 1 o or 12 feet distant. 

 The rarefaction occasioned by it caused a perceptible refraction, 

 to the distance of about J- of an inch from the side of the poker. 

 A letter seen more distant from it appeared as usual; within 

 that distance there was a faint image of it reversed ; and still 

 nearer to the poker was a second image direct, and as distinct 

 as the object itself, but somewhat smaller, as in Fig. 8, in 

 which a section of the atmosphere surrounding the poker is 

 represented. At the bottom and sides it is nearly circular; 

 but upwards the circular form is lost in undulations, occasioned 

 by the rapid ascent of the rarefied air. 



The greatest deviation produced in this case measured about 

 ■i a degree. 



Exper. 6. By a red-hot bar of iron, 30 inches long, the refrac- 

 tions were much greater, the extreme deviations amounting to 

 full 1 i degree. 



The refractions observed in these experiments may, at first 

 view, be thought greater than could be caused by difference of 



