142 ON A PHYSICAL OBSER.VATOEY. 



should divide Lis attention between tlie plienomeijii revealed by a criti- 

 cal and continued examination of tlic sun and the production of similar 

 phenomena in the laboratory. In this way European investigators have 

 arrived at most interesting results. 



Again, Vv'e know that the emanations from the sun, and probably 

 from the stars, differ essentially in character. There is, first, the em;s 

 nation known as light, which of itself consists of various rays, which 

 generally indicate the incandescence of substances, which give the 

 sensation of different colors, and those v.iiich, in their ordinary condition, 

 are impercei^tible to the eye, but vrhich may be perceived by that organ 

 after they have passed through certain liquids; next, the heat emana- 

 tion, which is also of different kinds; then the chemical emanation, by 

 which photographic impressions are i)roduced; and, lastly, the i)hospho- 

 rogenic emanation, which abounds also in the electric discharge, and 

 which produces the glow of the diamond and the luminosity of the com- 

 pounds of lime, barium, and other substances with sulphur. 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 mete 

 ors, requires i)eculiar instruments, and such as are not found, at present^ 

 in ordinary astronomical observatories. For example, the celestial phe- 

 nomena which address themselves to the sense of sight are studied by 

 means of refracting telescopes, as are, also, those of the photographic 

 ray, although this requires a i^eculiar form of lens, while the he;it-ra;> 

 of lower intensity and the phosphorogenic ray are not transmitted b.^ 

 glass; the former is readily converged to a focus by a lens of rock-sair, 

 and the latter by one of quartz. They may all, however, as in the case 

 of light, be concentrated into foci by metallic reflectors. 



In regard to terrestrial physics, the phenomena are also various, and 

 t!:e forces by which they are produced are constantly changing both in 

 ii: tensity and, in some cases, in direction. We now know that the 

 n;agnetism of the earth scarcely remains the same from one mom.ent to 

 another, and that these changes are connected with the appearance of 

 the aurora borealis and electrical discharges in the atmosphere. They, 

 also, in all probability, may ultimately be referred to disturbances pro- 

 duced by external inlluences, such as those from the sun, moon, and 

 ])lanets. Furthermore, we may now consider the whole earth as an 

 immense conductor charged with negative electricitTy-, of which the 

 intensity is in a continued state of change, and of which a knov.ledge of 

 the laws, as well as those of tlie changes of magnetism, is highly desira- 

 ble. For the proper study of these, continuous self-recording instru- 

 ments are necessary. 



There is also an important field of observation in regard to ordinary 

 meteorology, such as the changes of the pressure of the atmosphere, 

 and its connection with other phenomena; of the norma! and abnormal 

 wuuds; isolated currents of the atmosi)here, and especial!}' those of a. 



