Notices respecting New Books. 145 



practice. I may state in addition, that the colours produced by the 

 brown precipitate to which a small quantity of lime has been added, 

 resist the action of soap and acids, &c, to which all madder colours 

 must be subjected in order to heighten them, much better than if no 

 lime had been added. I therefore conclude, that though the possi- 

 bility in general of dyeing with madder is due to alizarin, the solidity 

 and brilliance of madder colours must be ascribed to rubiacin. 



XXII. Notices respecting New Books. 



Results of Astronomical Observations made during the years 1 834, 

 1835, 1836, 1837, 1838, at the Cape of Good Hope; being the 

 Completion of a Telescopic Survey of the whole Surface of the visible 

 Heavens, commenced in 1825. By Sir John F. W. Herschel, 

 Bart., K.H.,8fC.8(C. London : Smith, Elder and Co. 1847. (Third 

 notice.) 



Chap. III. Of Astrometry, or the Numerical Expression of the Appa- 

 rent Magnitudes of the Stars. 



r ¥^HE author commences the present chapter by remarking, that 

 ■* "a method of determining the relative magnitudes of the fixed 

 stars, that is, of assigning to each, at a given epoch, a number ex- 

 pressing on a certain scale the intensity of its light, has long been 

 and still remains a desideratum in astronomy." The inquiry is beset 

 with numerous and great difficulties, which have only been partially 

 met by any of the contrivances which have been applied to the pur- 

 pose . Some of the difficulties, indeed, would seem to be insuperable ; 

 for instance those which arise from a diversity of colour in the light 

 of the stars, since it appears hardly possible to assign any precise 

 meaning to the proportion of total brightness of two stars differing 

 sensibly in colour. Instances indeed occur where there would be no 

 hesitation in assigning the order of precedence ; but there are many 

 other cases in which different observers would probably always come 

 to opposite conclusions ; and " nothing short of a separate and inde- 

 pendent estimation of the total amount of the red, the yellow, and 

 the blue rays in the spectrum of each star would suffice for the reso- 

 lution of the problem of astrometry in the strictness of its nume- 

 rical acceptation ; and this the actual state of optical science leaves 

 us destitute of the means even of attempting with the slightest 

 prospect of success." We must therefore for the present, so far at 

 least as stars differing in colour are concerned, rest contented with 

 a somewhat rude and coarse approximation in assigning numerically 

 their places on a photometric scale ; and even with respect to those 

 which offer no marked peculiarity of colour, the results obtained by 

 the application of instrumental methods are in many instances so 

 strongly at variance with the plain judgement of the unaided eye, that 

 little is to be hoped for from such methods. It is very important, 

 however, to establish, if possible, by direct and extensive observa- 

 tions, a scale of magnitudes which shall be applicable, with some 

 Phil. Mag. S. 3. Vol. 33. No. 220. Aug. 1848. L 



