PHOTOMETIUC METHODS. 123 



Steinheil and Sir John Herschel. It will be sufficient for the 

 object of this work briefly to indicate the different methods. 

 These were a comparison of the shadows of artificial lights, 

 diifering in numbers and distance ; diaphragms ; plane glasses 

 of different thickness and colour ; artificial stars formed by 

 reflection on glass spheres ; the juxta-position of two seven-feet 

 telescopes, separated by a distance which the observer could 

 pass in about a second ; reflecting instruments in which two 

 stars can be simultaneously seen and compared, when the tele- 

 scope has been so adjusted that the star directly observed gives 

 two images of like intensity ; OT an apparatus having, (in front 



57 This is the application of reflecting sextants to the 

 determination of the intensity of stellar light; of this instru- 

 ment I made greater use when in the tropics than of the 

 diaphragms recommended to me by Borda. I began my in- 

 vestigation under the clear skies of Cumana, and continued 

 them subsequently till 1803, but under less favourable con- 

 ditions, on the elevated plateaux of the Andes, and on the 

 coasts of the Pacific, near Guayaquil. I had formed an arbi- 

 trary scale in which I marked Sirius, as the brightest of all 

 the fixed stars, equal to 100 ; the stars of the first magnitude 

 between 100 and 80, those of the second magnitude between 

 80 and 60, of the third between 60 and 45, of the fourth 

 between 45 and 30, and those of the fifth between 30 and 20. 

 I especially measured the constellations of Argo and Grus, in 

 which I thought I had observed alterations since the time of 

 Lacaille. It seemed to me after a careful combination of 

 magnitudes, using other stars as intermediate gradations, that 

 Sirius was as much brighter than Canopus, as a Centauri 

 than Achernar. My numbers cannot, on account of the above 

 mentioned mode of classification, be compared directly with 

 those which Sir John Herschel made public as early as 1838. 

 (See my Recueil d'Observ. astr., vol. i. p. Ixxi., and Relat. 

 hist, du Voyage aux Regions equin.^ t. i. pp. 518 and 624; 

 also Lettre de M. de Humboldt a M. Schumacher en Fevr. 1839, 

 in the Astr. Nachr., no. 374.) In this letter I wrote as follows: 

 "M. Arago, qui possede des moyens photometriques entiere- 

 ment differents de ceux qui out ete publics jusqu'ici, m'avait 

 rassurc sur la partie des erreurs qui pouvaient provenir du 



