1 86 SCIENTIFIC APPARATUS. 



with the thermal radiation from the celestial body under examina- 

 tion. For the spectroscopic examination of the heavenly bodies, 

 the spectroscope, an instrument which will be found described in 

 another section, is applied to the telescope in lieu of an eye-piece ; 

 the image, whether produced by an object-glass or a mirror, is 

 made to fall on the slit, in which case we get the spectrum ot 

 various portions of such bodies as the sun, moon, planets, comets, 

 aud nebulae without any difficulty, with the distinction that the 

 brighter the body the more dispersion can be employed in the 

 spectroscope. In the case of stars, as the spectrum of a point is 

 a line, it becomes necessary to widen out this line into a band, in 

 order to render the various absorption phenomena visible. For 

 this purpose a cylindrical lens is employed either in front of the 

 slit, according to the method used by Mr. Huggins, or nearer the 

 eye, according to that favoured by many continental astronomers. 

 Here, again, as in the case of ordinary eye observation, we may 

 replace the retina by a photographic plate, and obtain photographs 

 of the spectra of the various heavenly bodies as well as of the 

 heavenly bodies themselves. 



The polariscopic examination of the heavenly bodies becomes a 

 point of extreme importance when an eclipse of the sun has to be 

 observed, and we here have a neat means of determining the posi- 

 tion in space of the particles which are reflecting the light towards 

 the eye. For this purpose, all that is necessary is to introduce a 

 bi-quartz or a savart, in front of a Nicol in the eye-piece, and 

 the various polariscopic phenomena are seen in this case as they 

 are in the examination of a light source in the laboratory. 



The application of a thermo-electric pile in lieu of an eye-piece 

 to determine the amount of heat radiation from the heavenly 

 bodies was, I believe, first employed by Professor Henry in the 

 case of his determination of the various temperatures of the dif- 

 ferent portions of the sun. Since that time it has been applied to 

 the heat of the stars by Mr. Stone, and to the heat of the moon by 

 Lord Rosse, who exhibits the actual instrument employed, and 



