or outer mouth, we may call it. The observer may
therefore look at them there direct, merely by means of an eye-piece. A
small portion of the astronomer's head, it is true, then encroaches on
the tube; it forms a screen, and interrupts some incident rays. Still,
in a large telescope, the loss does not amount to half by a great deal;
which it would inevitably do if the small mirror were there.

Those telescopes, in which the observer, placed at the anterior
extremity of the tube, looks direct into the tube and turns his back to
the objects, were called by Herschel _front view telescopes_. In vol.
lxxvi. of the _Philosophical Transactions_ he says, that the idea of
this construction occurred to him in 1776, and that he then applied it
unsuccessfully to a ten-foot telescope; that during the year 1784, he
again made a fruitless trial of it in a twenty-foot telescope. Yet I
find that on the 7th of September 1784, he recurred to a _front view_ in
observing some nebulae and groups of stars. However discordant these
dates may be, we cannot without injustice neglect to remark, that a
front view telescope was already described in 1732, in volume vi. of the
collection entitled _Machines and Inventions approved by the Academy of
Sciences_. The author of this innovation is Jaques Lemaire, who has been
unduly confounded with the English Jesuit, Christopher Maire, assistant
to Boscovitch, in measuring the meridian comprised between Rome and
Rimini. Jaques Lemaire having only telescopes of moderate dimensions in
view, was obliged, in order not to sacrifice any of the light, to place
the great mirror so obliquely, that the image formed by its surface
should fall entirely outside the tube of the instrument. So great a
degree of inclination would certainly deform the objects. The _front
view_ construction is admissible only in very large telescopes.

I find in the _Transactions_ for 1803, that in solar observations,
Herschel sometimes employed telescopes, the great mirror of which was
made of glass. It was a telescope of this sort that he used for
observing the transit of Mercury on the 9th of November, 1802. It was
seven English feet long, and six inches and three tenths in diameter.

Practical astronomers know how much the mounting of a telescope
contributes to produce correct observations. The difficulty of a solid
yet very movable mounting, increases rapidly with the dimensions and
weight of an instrument. We may then conceive that H