256 Royal Astronomical Society : — Prof. Bessel o?i the 



February 13. — 'A^op^wra, No. 1. "On a case of Superficial 

 Colour presented by a Homogeneous Liquid internally coloui'less." 

 By Sir John Frederick William Herschel, Bart., F.R.S., &c. 



The author observed that a solution of sulphate of quinine in tar- 

 taric acid, largely diluted, although perfectly transparent and colour- 

 less when held between the eye and the light, or a white object, 

 yet exhibits in certain aspects, and under certain incidences of the 

 light, an extremely vivid and beautiful celestial blue colour, appa- 

 rently resulting from the action of the strata which the light first pe- 

 netrates on entering the liquid ; and which, if not strictly sujierficial, 

 at least exert their peculiar power of analysing the incident rays, and 

 dispersing those producing the observed tint, only through a very 

 small depth within the medium. The thinnest film of the liquid 

 seems quite as eftective in producing this superficial colour as a con- 

 siderable thickness. 



ROYAL ASTRONOMICAL SOCIETY. 



[Continued from p. 175] 



December 13, 1844. — Extract from the Translation of a Letter 

 from Professor Bessel, dated Konigsberg, lOtli of August. 1844. On 

 the Variations of the Proper Motions of Procyon and Sirius. Com- 

 municated by Sir J. F. W. Herschel. 



The subject which I wish to communicate to you, seems to me 

 so important for the whole of practical astronomy, that I think it 

 worthy of having your attention directed to it. I find, namely, that 

 existing observations entitle us without hesitation to affirm that the 

 jjroper motions, of Procyon in declination, and of Sirius in right as- 

 cension, are not constant ; but, on the contrary, that they have, 

 since the year 1755, been very sensibly altered. If this be so, the 

 observations of the place of a star at two epochs are no longer suffi- 

 cient to express its place for any indefinite time ; but, for this pur- 

 pose, it is necessary to investigate the law of the change. It follows 

 also from this, that we are yet very far off from the correctness we 

 imagined ourselves to have arrived^atin the fundamental determina- 

 tions of astronomy ; and, that a new problem presents itself, whose 

 solution will cost much labour and a long period of time, viz. the 

 problem of determining the special motions of a star. For, even if 

 a change of the motion, can, up to the present time, be proved only 

 in two cases, yet will all other cases be rendered thereby liable to sus- 

 picion : and it will be equally difficult, by observations, to free other 

 proper motions from the suspicion of change, and to get such a 

 knowledge of the change as to admit of its amount being calculated. 



The earliest suspicion of the want of constancy of the proper 

 motion was derived about the year 1834, from the corrections of 

 the clock-time, which, at this observatory, were registered with 

 every observed culmination of a fixed star and its reduction to the 

 meridian. At that time it began to be remarked in a striking man- ■ 

 ner, that negative clock-corrections derived from Sirius Avere greater, 

 and positive corrections less, than those resulting from the other 

 fundamental stars. 



