92 Mr Herschel on Double and Multiple Stars. 



of the light ; and he cites some experiments, by which it ap- 

 pears that the light of an average star of the first magnitude is at 

 least 150 times that of the sixth. He then adduces a series of 

 observations of a considerable number of the closer stars of M. 

 Struve^s catalogue, by which it appears that the Slough tele*. 

 scope easily defines with its ordinary sweeping power the ge- 

 nerality of M. Struve's stars of the first class, and many, of 

 those marked by him as vicince, and exenpervicince ; but those 

 which have the epithet vicinissirncE, he has not yet succeeded 

 in separating with the highest power (240) usually applied, — 

 which indeed was to be expected. In lieu of M. Struve"^ clas- 

 sification of double stars, which he considers as enlarging be- 

 yond due limits the number of those of the first class, he pro- 

 poses the following system, which in fact very nearly approxi- 

 mates to that originally followed by Sir William Herschel. 

 J close - - - - 0" and below 1" 



( not close - - - 1 and below 2 

 Class II. ------ 2 and below 4 



Class III. - - - - - 4 and below 8 



Class IV. 8 and below 16 



Class V. 16 and below 32 



Class VI 32 and below 64 



So that the limit of distance of stars of the nth class shall be 

 2" X V\ 



" The author then subjoins a list of stars common to his 

 two former catalogues, and to that of Professor Struve, 86 

 in number ; after which he proceeds to describe some singular 

 phenomena observed in the course of his examination of these 

 objects, which explain certain discrepancies between the re- 

 sults of observations of their angles of position on different 

 nights, and which tend to throw light on some obscure points 

 in the theory of vision. He considers it as rendered very pro- 

 bable by some of the facts adduced, that time is required for 

 light to make an impression on the retina, as well as for the im- 

 pression made to wear off; and that this time is the less, the 

 brighter the object; and explains by this principle a remarkable 

 degree of unsteadiness and fluctuation observed in the limb of 

 the planet Mars, while small stars in the field remained per- 

 fectly tranquil, as well as certain other curious phenomena. 



