Mr. C. J. Monro on a case of Stereoscopic Illusion, 15 



rature had risen to 47°*8. Not being in possession of corre- 

 sponding afternoon observations, I am not able to fix the time 

 of greatest temperature ; I am inclined to think, however, that, 

 as in the case of the air, it occurred about noon. 



Ramond, in the thirty-five ascents of the Pic du Midi made 

 by him in the course of twelve years in spring, summer, and 

 winter, collected all the plants of the terminal cone, whose 

 height is 16 metres, and whose base covers several ares; he 

 there observed 71 phanerogamous plants. I noticed 131 such 

 plants on the terminal cone of the Faulhorn, whose height is 

 80 metres, and whose base covers 4 hectares 50 ares (about 

 11 English acres), the summit being 2683 metres above the 

 level of the sea. According to the recent Swedish explora- 

 tions, and the account of previous expeditions given by M. 

 Malmgren*, the whole archipelago of Spitzbergen only con- 

 tains 93 such plants. Independently of the original geogra- 

 phical distribution, the temperature of the soil is sufficient 

 to account for the number and the variety of the species which 

 vegetate on the summits of the Alps and of the Pyrenees ; they 

 are there heated more by the soil which bears, than by the air 

 which surrounds them, and their respiratory functions are stimu- 

 lated by a strong light. On the other hand, at Spitzbergen, 

 notwithstanding the continual presence of the sun above the 

 horizon during summer, the heat of the sun's rays, absorbed 

 almost entirely by the vast thickness of atmosphere they have 

 traversed, is incapable of raising the temperature of the soil 

 above that of the air. The soil remains always frozen at a depth 

 of a few decimetres ; and vegetation not being stimulated either 

 by the heat of the air or by that of the soil, the entire flora of 

 the region is limited to a small number of plants capable of living 

 and flourishing at a temperature but a few degrees above zero. 



III. On a case of Stereoscopic Illusion. By C. J. Monro f. 



IF the phenomenon here to be described has not been described 

 before, it is perhaps wortb describing for its own sake. If 

 it is well known, it is worth while to point out its bearing on a 

 recently contested point in the theory of binocular vision. 



In his separate work on the Stereoscope, published in 1856, 

 Sir David Brewster maintained (pp. 78, 81) that when we see or 

 seem to see an object single and solid with two eyes, it is because 

 we see any point with both eyes at the intersection of the two 



* Petermann's Mittheilungen, 1863, p. 48. 

 t Communicated by the Author. 



