ce-penetrating Power of
Telescopes_.

This memoir contains excellent things; still, it is far from exhausting
the subject. The author, for instance, entirely overlooks the
observations made by day. I also 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 astronomer 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.

FOOTNOTES:

[18] Conforming to general usage, and to Sir W. Herschel himself, we
shall allude to this instrument as the _forty-foot_ telescope, though M.
Arago adheres to thirty-nine feet and drops the inches, probably because
the Parisian foot is rather longer than the English.--_Translator's
Note_.

[19] It would be more correct to say four times _as much_
light.--_Translator_.

[20] On comparing the Cassegrain telescopes with a small convex mirror,
to the Gregorian telescopes with a small concave mirror, Captain Kater
found that the former, in which the luminous rays do not cross each
other before falling on the small mirror, possess, as to intensity, a
marked advantage over the latter, in which this crossing takes place.

[21] In the selection of _i_ Bootis as a test, Arago has taken the
precaution of giving its corresponding denomination in other catalogues,
and Bailey appends the following note, No. 2062, to 44 Bootis. "In the
British Catalogue this star is not denoted by any letter: but Bayer
calls it _i_, and on referring to the earliest MS. Catalogue in MSS.
vol. xxv., I find it is there so designated; I have therefore restored
the letter." (See Bailey's Edition of Flamsteed's British Catalogue of
Stars, 1835.) The distance between the two members of this double star
is 3".7 and position 23 deg..5. See "Bedford Cycle."--_Translator_.

LABOURS IN SIDEREAL ASTRONOMY.

The curious phenomenon of a periodical change of intensity in certain
stars, very early excited a keen attention in Herschel. The first memoir
by that illustrious observer presented to the Royal Society of