92 COSMOS. 



appeared stars in Cygnus and Ophiuchus (tie former of wh'.ch 

 continued luminous for twenty-one years), with the bright- 

 ness of other stars, called attention to photometric determina- 

 tions. The so-called dark stars of Ptolemy, which were he- 

 low the sixth magnitude, received numerical designations 

 according to the relative intensity of their light. " Magni- 

 tudes, from the eighth down to the sixteenth," says Sir John 

 Herschel, " are familiar to those who are in the prajCtice of 

 using powerful instruments.^ But at this fciint degree of 

 brightness, the denominations for the difierent gradations in 

 the scale of magnitudes are very undetermined, for Struve 

 occasionally classes among the twelfth or thirteenth stars 

 which Sir John Herschel designates as belonging to the 

 eighteenth or twentieth magnitudes. 



The present is not a fitting place to discuss the merits of 

 the very difierent methods which have been adopted for the 

 measurement of light within the last hundred and fifty years, 

 from Auzout and Huygens to Bouguer and Lambert ; and 

 from Sir William Herschel, Humford, and Wollaston, to Stein- 

 heil and Sir John Herschel . It will be sufficient for the ob- 

 ject of this work briefly to indicate the difierent methods. 

 These were a comparison of the shadows of artificial lights, 

 differing in numbers and distance ; diaphragms ; plane-glass- 

 es of different thickness and color ; artificial stars formed by 

 reflection on glass spheres ; the juxtaposition 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 telescope has been so adjusted that the star directly ob- 

 served gives two images of like intensity ;t an apparatus hav» 



* Sir John Herschel, Outlines of Astr., p. 520-27. 



t This is the apphcatiou of redacting sextants to the determination 

 of the intensity of stellar light; of this instrument I made greater use 

 when in the tropics than of the diaphragms recommended to me by 

 Borda. I began my investigation under the clear skies of Cumana, and 

 continued them subsequently till 1803, but under less Divorable condi- 

 tions, on the elevated plateaux of the Andes, and on the coasts of the 

 Pacific, near Guayaquil. I had formed an arbitrary scale, in which 1 

 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 CO, of the third between %^ and 45, of the 

 fourth between 45 and 3C, 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 LacaiUe. It seemed 

 to me, after a careful combination of magnitudes, using other stars aa 

 intermediate gradations, that Sirius was as much brighter than CanopuSf 

 aa a Centauri than Achernar. My numbers can not, on account of tho 



