150 ANNUAL OP SCIENTIFIC DISCOVERT. 



wliich pertain niotals possess of arrestinij the calorific rays, Avhile 

 they allow the hiiiiinous rays to pass through. Silver, when de- 

 posited by a partieiilar chiMiiieal j)r()cess in very thin laAcrs, ])os- 

 sesses tliis proi)erty in a hii^h desjree. M. Foueanlt has slieathed 

 the objective of a telescope with a layer of this metal, and there is 

 produced at the focus of the instrument an image perfectly clear 

 and ai::reeal)le to the eye. It exactly resembles one which a 

 violet-colored glass would produce. 



This discovery of M. Foucault has Ijeen pronounced by M. Le 

 Verri(!r to be of the highest possible imi)()rtance. M. Foucault's 

 experiment was made upon a telescope with a very small oi)jec- 

 tive. Since then, further experiments have been made on one of 

 nine inches, Avliieh were <iuite satisfactcny. The solar i"a3's re- 

 fracted by the objective sheathed with llu; metal liave a very pecu- 

 liar I)lui>h tint, which made M. Woltf imagine that a considerable 

 proportion of the calorilie rays might possii)ly have Ijecm elimi- 

 nated. The I'ays were examined with a spectroscope, and were 

 found to be dcjjrived of their extra redness, and inclined to be of 

 a very deep blue color. Clearly, the calorilie rays had been 

 sto])pi'd in their passage. Theory thus aflbrded the most brilliant 

 conlirmation t)f experience. 



A large objective at the Observatory of Paris, which was in 

 process of construction, an'ordcd an excellent op])ortunity for ex- 

 l)eriment. 1'he exterior surlace of the glass was duly silvered, 

 and, on turning towards the sun, the image was presented devoid 

 almost entirely of its heat. The layer of silver in no way interfered 

 with the optical properties of the glass. All the numerous details 

 which the most experienced observers have detected in sun-spots 

 were at once visible. "The entire surface of the sun appeared 

 covered with an irregular stippling, the constituents of which 

 were of dilVerent sizes, and grouped in constellations of vai'ious 

 forms." "In pro])ortion," says M. Le Verrier, " as we see the 

 image better, all idea of a regular structure vanishes; nor is 

 there any indication of such a one as would result from the ag- 

 glomeration of identical elements placed in juxtaposition or dove- 

 tailed with each other. At some moments, the clearness is such 

 as to promise the analysis of the shaded portions, and make us 

 long to have recourse to more and more powerful instruments." 

 M. Fhimmarion, however, admits that the medium does throw 

 some kind of veil over the object investigated. — Header. 



TEMPERATURE AT GREAT ELEVATIONS. 



Mr. Glaisher has given, in a lecture at the Royal Institution, a 

 resume of his seientilie ex})eriments in balloons. Tables, record- 

 ing the decline of temperature with elevation, show that when the 

 sky was clear, a more rapid decline took place than when the sky 

 was cloudy. Under a clear sky, a fall of 1° takes place within 100 

 feet of the earth ; but at heights exceeding 25,000 feet, it is neces- 

 sary to pass through 1,000 feet of vertical height to obtain a fall 

 of 1° in temiierature. At extreme elevations, in both states of the 

 sky, the air became very dry, but, as far as his experiments went, 



