gin and crossed each other at very acute
angles. There seems to be nothing, then, in the difference of the
results at which we need to be much surprised.

Herschel increased the catalogue, already so extensive, of the mysteries
of vision, when he explained in what manner we must endeavour to
distinguish separately the two members of certain double stars very
close to each other. He said if you wish to assure yourself that _e_
Coronae is a double star, first direct your telescope to _a_ Geminorum,
to _z_ Aquarii, to _m_ Draconis, to _r_ Herculis, to _a_ Piscium, to _e_
Lyrae. Look at those stars for a long time, so as to acquire the habit of
observing such objects. Then pass on to _x_ Ursae majoris, where the
closeness of the two members is still greater. In a third essay select
_i_ Bootis (marked 44 by Flamsteed and _i_ in Harris's maps)[21], the
star that precedes _a_ Orionis, _n_ of the same constellation, and you
will then be prepared for the more difficult observation of _e_ Coronae.
Indeed _e_ Coronae is a sort of miniature of _i_ Bootis, which may itself
be considered as a miniature of _a_ Gem. (_Philosophical Transactions_,
1782, p. 100.)

As soon as Piazzi, Olbers, and Harding had discovered three of the
numerous telescopic planets now known, Herschel proposed to himself to
determine their real magnitudes; but telescopes not having then been
applied to the measurement of excessively small angles, it became
requisite, in order to avoid any illusion, to try some experiments
adapted to giving a scale of the powers of those instruments. Such was
the labour of that indefatigable astronomer, of which I am going to give
a compressed abridgment.

The author relates first, that in 1774, he endeavoured to ascertain
experimentally, with the naked eye and at the distance of distinct
vision, what angle a circle must subtend to be distinguished by its form
from a square of similar dimensions. The angle was never smaller than 2'
17"; therefore at its maximum it was about one fourteenth of the angle
subtended by the diameter of the moon.

Herschel did not say, either of what nature the circles and squares of
paper were that he used, nor on what background they were projected. It
is a lacuna to be regretted, for in those phenomena the intensity of
light must be an important feature. However it may have been, the
scrupulous observer not daring to extend to telescopic vision what he
had discovered relative to vision with the n