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 Herschel’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 
