LABOURS IN SIDEREAL ASTRONOMY. 285 



find, that the hypothetical part of the discussion is not 

 perhaps so distinctly separated from the rigorous part as 

 it might be ; that disputable numbers, though given with 

 a degree of precision down to the smallest decimals, do 

 not look well as terms of comparison with some results 

 which, on the contrary, rest on observations bearing 

 mathematical evidence. 



Whatever may be thought of these remarks, the astron- 

 omer or the physicist who would like again to undertake 

 the question of visibility with telescopes, will find some 

 important facts in Ilerschel's memoir, and some ingenious 

 observations, well adapted to serve them as guides. 



LABOURS IN SIDEREAL ASTRONOMY. 



The curious phenomenon of a periodical change of in- 

 tensity in certain stars, very early excited a keen attention 

 in Herschel. The first memoir by that illustrious observer 

 presented to the Royal Society of London and inserted 

 in the Philosophical Transactions treats precisely of the 

 changes of intensity of the star o in the neck of the 

 Whale. 



This memoir was still dated from Bath, May, 1780. 

 Eleven years after, in the month of December, 1791, 

 Herschel communicated a second time to that celebrated 

 English Society the remarks that he had made by some- 

 times directing his telescopes to the mysterious star. At 

 both those epochs the observer's attention was chiefly 

 applied to the absolute values of the maxima and minima 

 of intensity. 



The changeable star in the Whale was not the only 

 periodical star with which Herschel occupied himself. 

 His observations of 1795 and of 1796 proved that a Her- 

 culis also belongs to the category of variable stars, and 



