462 WRITINGS OF JOSEPH HENRY. [1870 



indicated, to investigate the nature and changes of the con- 

 stitution of the heavenly bodies ; to study the various ema- 

 nations from these in comparison with the results of experi- 

 ments, and to record and investigate the different phenomena 

 which are included under the general term of terrestrial 

 physics. 



A wide field has been opened for the study of the nature 

 of the sun and other heavenly bodies by the application of 

 the spectroscope, different modifications of the telescope, and 

 other lately invented appliances. We now know that the 

 sun is undergoing remarkable changes, the character of 

 which can only be ascertained by the results of accurate 

 observations compared with those of experimental investi- 

 gation. The observer should divide his attention between 

 the phenomena revealed by a critical and continued exami- 

 nation of the sun and the production of similar phenomena 

 in the laboratory. In this way European investigators have 

 arrived at most interesting results. 



Again, we know that the emanations from the sun, and 

 probably from the stars, differ essentially in character. 

 There is first, the emanation known as light, which of itself 

 consists of various rays (generally indicating the incan- 

 descence of substances,) that give the sensation of different 

 colors, some of which though in their ordinary condition 

 imperceptible to the eye, may be perceived by that organ 

 after they have passed through certain liquids; next, the 

 heat emanation, which is also of different kinds ; then 

 the chemical emanation, by which photographic im- 

 pressions are produced ; and lastly, the phosphorogenic 

 emanation (abounding also in the electric discharge), that 

 produces the temporary glow of the diamond and the 

 luminosity of the compounds of lime, barium, and other 

 substances with sulphur, when taken into the dark. To 

 study these or other emanations as they may appear 

 in the fixed stars, or are reflected from the moon and 

 planets, or as they may be found in the aurora borealis, the 

 zodiacal light, and in shooting-stars or larger meteors, re- 

 quires peculiar instruments, and such as are not found, at 



