75 Notices respecting New Books. 



tini in 1844-46, and again by Smyth in 1849-51. The two lists of 

 English observations agree well enough with each other, but differ 

 in a remarkable degree, and in a large number of instances, from 

 that of the observations made in Italy. Some of the discrepancies 

 are adverted to by Signor Sestiniin these terms : — " Now, beginning 

 with the comparison of y Andromedae, we have Smyth emerald- 

 green, and Sestini white ; but Herschel and Struve at another date 

 call it azure. Moreover, observing it again after a lapse of two 

 years, and four years after Smyth, I find it no longer white, but a 

 strong blue!" "Now see B(95) Herculis : according to Smyth, 

 one is greenish and the other red ; but we think them both a golden- 

 yellow ; A Ophiuchi, by Smyth, one ruddy and the other pale yel- 

 low ; but we take them to be both orange. The contrary occurs in 

 t Bootis, the components of which by Smyth are both pale yellow ; 

 but we deem one to be orange and the other azure." 



What can be the cause of such differences ? " The disagreements 

 between the tints of stars as given by Sir William Herschel and 

 myself," says Captain Smyth, " are partly accounted for by his pe- 

 culiarity of vision and the tone of metal in his reflectors. But I am 

 at a loss why refractors should diflfer so widely as here shown ; and 

 therefore hope the subject will be more closely pursued than it has 

 hitherto been." The explanation proposed by Signor Sestini, viz. 

 that the colours of the stars vary in consequence of variations of 

 their velocity, is not admissible. Neither are we prepared to take 

 the view which Captain Smyth appears to advocate, viz. that very 

 minute differences in the velocity of transmission of rays of different 

 colour, cause variations of the colours of stars. Certainly on this 

 hypothesis, if a new star suddenly appeared in the sky, its existence 

 would be announced at successive epochs by the different rays of its 

 spectrum, and its colour would not be permanent till the rays had 

 all reached our position in space. Changes of colour in the reverse 

 order would occur on the extinction of a star. On the same hypo- 

 thesis, variations of colour would accompany variations of bright- 

 ness. But such variation of colour has not hitherto been detected 

 in stars that notably vary in brightness. It seems probable that the 

 discrepancies in the estimates of the colours of stars are due to 

 various sources of error in judging of tints, which after all form but 

 a small portion of their total light ; such, for instance, as the 

 general state of the atmosphere at the time of observation; the 

 effect of altitude above the horizon ; the effect on the eye of 

 the observer of the artificial light used for the purposes of obser- 

 ving. Possibly, also, the achromatism of the object-glass, which, 

 being adapted to the solar spectrum, may not be suitable to the 

 spectrum of the star, ought to be taken into account ; as well as the 

 necessity of a nice adjustment of the eye-piece for ehciting the proper 

 colour of each star. " Chromatic personal equation," that is, the fa- 

 culty in a greater or less degree of appreciating differences of colour, 

 must be a fruitful source of discrepancy. Many valuable hints are 

 given by Captain Smyth (jip. 306-310) towards obviating some of 

 these sources of error, and towards fixing upon a standard scale of 



