PHOTOMETRY. 125 



image of the moon or of Jupiter, and concentrates it through 

 a lens at different distances into a star more or less bright. 

 Sir John Herschel, who has been more zealously engaged than 

 any other astronomer of modern times in making numerical 

 determinations in both hemispheres of the intensity of light, 

 confesses that the practical application of exact photometric 

 methods must still be regarded as a " desideratum in astronomy ," 

 and that "photometry is yet in its infancy." The increasing 

 interest taken in variable stars, and the recent celestial phe- 

 nomenon of the extraordinary increase of light exhibited in 

 the year 1837 in a star of the constellation 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 lustre, without numerical estimates 

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

 Herschel in his Manual of Scientific Enquiry 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. 69 The first numerical scale, 

 based on estimates calculated M'ith the naked eye, but im- 



69 Compare for the numerical data of the photometric results 

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

 b) pp. 367-371; c) p. 440; and d) in his Outlines ofAstr.,p-p. 522 

 -525, 645-646. For a mere arrangement without numbers 

 see the Manual of Scientific Enquiry prepared for the use of the 

 Navy, 1849, p. 12. In order to improve the old conventional 

 mode of classing the stars according 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. 



