MIRAGE 321 



STATE OF THE ATMOSPHERE WHICH PRODUCES THE FORMS OF 

 MIRAGE OBSERVED BY VINCE AND BY SCORESBY. 



(From Nature, Vol. xxviil, May 24, 1883.) 



IN 1 88 1, when I wrote the article Light for the Encyc. Brit., I had not been 

 able to meet with any detailed calculations as to the probable state of the atmo- 

 sphere when multiple images are seen of objects situated near the horizon. I had 

 consulted many papers containing what are called "general" explanations of the 

 phenomena, but had found no proof that the requisite conditions could exist in 

 nature : except perhaps in the case of the ordinary mirage of the desert, where it is 

 obvious that very considerable temperature-differences may occur in the air within a 

 few feet of the ground. But this form of mirage is essentially unsteady, for it 

 involves an unstable state of equilibrium of the air. In many of Scoresby's obser- 

 vations, especially that of the solitary inverted image of his father's ship (then thirty 

 miles distant, and of course far below the horizon), the details of the image could be 

 clearly seen with a telescope, showing that the air must have been in equilibrium. 

 The problem seemed to be one well fitted for treatment as a simple example of 

 the application of Hamilton's General Method in Optics, and as such I discussed it. 

 The details of my investigation were communicated in the end of that year to the 

 Royal Society of Edinburgh, and will, I hope, soon be published. The paper itself 

 is too technical for the general reader, so that I shall here attempt to give a sketch 

 of its contents in a more popular form. But a curious little historical statement 

 must be premised. 



It was not until my calculations were finished that I found a chance reference 

 to a great paper by Wollaston (Phil. Trans. 1800). I had till then known only of 

 Wollaston's well-known experiment with layers of different liquids in a small vessel. 

 But these, I saw, could not reproduce the proper mirage phenomena, as the rays 

 necessarily enter and emerge from the transition strata by their ends and not by their 

 lower sides. This experiment is by no means one of the best things in Wollaston's 

 paper, so far at least as the immediate object of the paper is concerned. That so much 

 has been written on the subject of mirage during the present century, with only a casual 

 reference or two to this paper, is most surprising. It may perhaps be accounted for 

 by the fact that Wollaston does not appear to have had sufficient confidence in his own 

 results to refrain from attempting, towards the end of his paper, a totally different 

 (and untenable) hypothesis, based on the effects of aqueous vapour. Be the cause 

 what it may, there can be no doubt that the following words of Gilbert were amply 

 justified when they were written, early in the present century: "In der That ist 

 Wollaston der Erste und Einzige, der die Spieglung aufwarts mit Gliick zu erklaren 

 unternommen hat." For his methods are, in principle, perfectly correct and sufficiently 

 comprehensive ; while some of his experiments imitate closely the state of the air 

 requisite for the production of Vince's phenomena. Had Wollaston only felt the 

 necessary confidence in his own theory, he could hardly have failed to recognise that 

 what he produced by the extreme rates of change of temperature in the small air- 

 T. 41 



