82 SPECIAL RESULTS IN THE URANOLOGICAL 



from Auzout and Huygens to Bouguer and Lambert ; and 

 from William Herschel, Rumford, and Wollaston, to Stein- 

 heil and John Herschel. It is sufficient, according to the 

 object of this work, to notice them in a brief and general 

 manner. They include comparison with the shadows of 

 artificial lights differing in number and distance ; diaphragms ; 

 plane glasses of different thicknesses and colours ; artificial 

 stars formed by reflection on glass globes ; two 7 feet 

 telescopes so placed that the observer could pass from 

 one to the other in scarcely more than a second of time ; 

 reflecting instruments, in which two stars which were to be 

 compared could be seen at once, after the telescope had 

 been so arranged that the star which was seen direct had 

 given two images of equal intensity ;( 156 ) apparatuses with 

 a mirror adapted to the objective, and with shades, the 

 degree of intensity being measurable on a ring ; tele- 

 scopes with divided object-glasses, each half of which 

 received the star's light through a prism; and astro- 

 meters, ( 157 ) in which the image of the Moon, or of Jupiter, 

 is reflected by a prism, and this image is concentrated by a 

 lens, at different distances, into a brighter or fainter star. 

 The distinguished astronomer, who in modern times has 

 most zealously pursued in both hemispheres the numerical 

 determination of the intensity of light in different stars, 

 Sir John Herschel, owns, notwithstanding what he has him- 

 self accomplished, that the practical application of exact 

 photometric methods must still be regarded as a " deside- 

 ratum in astronomy," and that " photometry or the measure- 

 ment of light is still in its infancy." The increasing inte- 

 rest taken in variable stars, and a new cosmical event in 

 the extraordinary augmentation of light in a star of the ship 



