572 PROFESSOR TAIT ON MIRAGE. 
tion and by long series of exact measurements of the phenomena as seen by 
Marutev and Brot at Dunkirk, and by Araco and Biot at Majorca. The 
previous work of GRUBER, WOLTMANN, Buscu, and others, is carefully sum- 
marised by GILBERT in vol, xi. of his Annalen (1802) in notes to his translation 
of WoLLAsTON’S great paper of 1800. A good deal of Brot’s work is thus seen 
to have been anticipated. It may be well to quote here GiLBeRt’s remark as 
to the priority of explanation of some of these phenomena—think of it now as 
we may :-— 
“In der That ist WoLLAsSToN der Erste und Einzige, der die Spieglung aufwarts mit 
Gliick zu erkliren unternommen hat, ob er gleich auch hierin noch sehr viel zu thun iibrig lasst.” 
Biot, on the other hand, gives WoLLAsToN credit only for the physical, as dis- 
tinguished from the mathematical, parts of his paper. He says :— 
“Sous le rapport de la physique, son travail ne laisse rien & désirer.” 
Brot has considered the subject from a point of view somewhat similar to 
that which I had adopted, and anticipated of course the great majority of the 
more general results at which I had arrived. I was occasionally almost 
startled as I looked through his memoir, to find how closely (even in mode of 
stating them) I had reproduced some of his main ideas. His whole treatment, 
for instance, of the ordinary mirage of the desert:—on the assumption that the 
square of the velocity of a luminous corpuscle is proportional to the height 
above the ground, but only through a limited stratum, together with the 
important effects of limitation of the stratum:—-is almost the same as mine, 
except that he (inconveniently I think) uses the caustics in preference to the 
curve of vertices, though he also notices the latter as the courbe des minima. 
In consequence, I had all but made up my mind to withdraw my paper, before 
I had looked more than half-way through Bror’s long memoir ; for, though I 
found here and there statements which I think inaccurate, these are of very 
small consequence compared with the whole. But it was otherwise when I 
read farther, where Biot gives his tentative explanation of VINCE’s observation. 
There I found our assumptions to be so entirely different in character that, 
being fairly satisfied with my own, I thought I might still reasonably produce 
them with their results. My paper, therefore, appears as it was presented to 
the Society, except in so far as (a) a part of the introduction, (0) the detailed 
examination of the ordinary mirage of the desert, (¢) a discussion of the 
singular outline sometimes presented by the setting sun, and (d) a few minor 
remarks, are concerned. These parts have been simply struck out, the first as 
historically imperfect, the others as practically a mere reproduction of what 
had already been satisfactorily done by Brot, who had many opportunities of 
observing and measuring the phenomena. As to the ordinary mirage, however, 
there can be no doubt that the discovery of the existence of fou images, when 
