94 COSMOS. 



eideratum in astronomy," and that " photometry is yec /*. * 

 infancy." The increasing interest taken in variable swrs, 

 and the recent celestial phenomenon of the extraordinary in- 

 crease of light exhibited in the year 1837 in a star of the con- 

 stellation Argo, has made astronomers more sensible of the 

 importance of obtaining certain determinations of light. 



It is essential to distinguish between the mere arrangement 

 of stars according to their luster, without numerical estimates 

 of the intensity of light (an arrangement adopted by Sir John 

 Herschel in his Manual of Scientific Inquiry prepared for 

 the Use of the Navy), and classifications in which intensity 

 of light is expressed by numbers, under the form of so-called 

 relations of magnitude, or by more hazardous estimates of the 

 quantities of radiated light.* The first numerical scale, based 

 on estimates calculated with the naked eye, but improved by 

 an ingenious elaboration of the materials! probably deserves 

 the preference over any other approximative method practi- 

 cable in the present imperfect condition of photometrical in- 

 struments, however much the exactness of the estimates must 

 be endangered by the varying powers of individual observers 

 the serenity of the atmosphere the different altitudes of 

 widely-distant stars, which can only be compared by means 

 of numerous intermediate stellar bodies and above all by the 

 unequal color of the light. Very brilliant stars of the first 

 magnitude, such as Sirius and Canopus, a Centauri and Acher- 

 nar, Deneb and Vega, on account of their white light, admit 

 far less readily of comparison by the naked eye than fainter 

 stars below the sixth and seventh magnitudes. Such a com- 

 parison is even more difficult when we attempt to contrast 

 yellow stars of intense light, like Procyon, Capella, or Atair, 

 with red ones, like Aldebaran, Arcturus, and Betelgeux.J 



* Compare, for the numerical data of the photometric results, four 

 tables of Sir John Herschel's Astr. Obs. at the Cape, a), p. 341 ; b), p. 

 367-371 ; c), p. 440 ; and d), in his Outlines of Astr., p. 522-525, 645- 

 646. For a mere arrangement without numbers, see the Manual of 

 Scientific Inquiry prepared for the Use of the Navy, 1819, p. 12. In 

 order to improve the old conventional mode of classing the stars accord- 

 ing to magnitudes, a scale of photometric magnitudes, consisting in the 

 addition of 0-41, as explained more in detail in Astr. Obs. at the Cape, p. 

 370. has been added to the vulgar scale of magnitudes in the Outlines of 

 Astronomy, p. 645, and these scales are subjoined to this portion of the 

 present work, together with a list of northern and southern stars. 



t Argelander, Durchmusterung des nordl. Himmels zwischen 45 und 

 80 Decl. 1846, s. xxiv.-xxvi. ; Sir John Herschel, Astr. Obscrv. al the 

 Cape of Good Hope, p. 327, 340, 365. 



t Op. cit., p. 304, aud Outl., p. 522. 



