208 . ANNUAL OF SCIENTIFIC DISCOVERY. 



a proper medium, cause the molecules of that medium to oscillate in 

 slower periods than those of the incident waves. In this case, there- 

 fore, the invisible rays are rendered visible by the lowering of their 

 refrangibility ; while in the experiments of Prof. Tyndall, the ultra- 

 red rays are rendered visible by the raising of their refrangibility. 

 To the phenomena brought to light by Prof. Stokes, the term fluores- 

 cence has been applied by their discoverer, and to the phenomena 

 brought forward Prof. Tyndall proposes to apply the term calores- 

 cence. 



It was the discovery more than three years ago, of a substance 

 opaque to light, and almost perfectly transparent to radiant heat, a 

 substance which cut the visible spectrum of the electric light sharply 

 off at the extremity of the red, and left the ultra-red radiation 

 almost untouched, that led Proi Tyndall to the foregoing results. 

 They lav directly in the path of his investigation, and it was only the 

 diversion of his attention to subjects of more immediate interest that 

 prevented him from reaching much earlier the point which he has 

 now attained. On this, however, Prof. Tyndall can found no claim, 

 and the idea of rendering ultra-red rays visible, though arrived at in- 

 dependently, docs not by right belong to him. The right to a scien- 

 tific idea or discovery is secured by the act of publication, and, in 

 virtue of such an act, priority of conception as regards the conversion 

 of heat-rays into light-rays, belongs indisputably to Dr. Akin. At the 

 meeting of the British Association, assembled at Newcastle in 18G3, 

 he proposed three experiments by which he intended to solve this 

 question. He afterwards became associated with an accomplished 

 man of science, Mr. Griffith, of Oxford, and jointly with him pursued 

 the inquiry. Two out of the three experiments proposed at New- 

 castle by Dr. Akin are quite impracticable. In the third it was pro- 

 posed to concentrate by a large burning mirror the rays of the sun, to 

 cut off the luminous portion of the radiation by " proper absorbents," 

 and then to operate with the obscure rays. Dr. Akin employed in 

 his experiments a mirror 36 inches in diameter, but he has hith- 

 erto failed to realize his idea. With a mirror four inches in diam- 

 ter, the radiant source with which his researches had rendered him 

 familiar, and a substance which he had himself discovered to filter the 

 beam of the electric lamp, Prof. Tyndall obtained all the results above 

 described. London Reader. 



EFFECT OF THE ATMOSPHERE ON. LIGHT. 



M. Janssen has published the result of his researches, showing the 

 power of elective absorption which the terrestrial atmosphere exer- 

 cises on light. This action, he says, is represented by fine numerous 

 sombre rays on the spectrum of all the light, the rays of which have 

 traversed a sufficient thickness of our atmosphere. In 1833, Sir David 

 Brewster announced that the solar spectrum at the horizon presented 

 new dark bands, which disappeared as the sun rose, but did not at- 

 tribute their disappearance to the terrestrial atmosphere. M. Janssen, 

 however, affirms that these bands resolve themselves into fine lines 

 comparable to the solar rays properly so called ; and that they are 

 always visible in the spectrum, and vary only in intensity according 

 to the hight of the sun. He finds that in the red and orange parts of 



