reflecting telescopes. They
thought that the image of a small circle cannot be distinct, cannot be
sharp at the edges, unless the pencil of rays coming from the object in
nearly parallel lines, and which enters the eye after having passed
through the eye-piece, be sufficiently broad. This being once granted,
the inference followed, that an image ceases to be well defined, when it
does not strike at least two of the nervous filaments of the retina with
which that organ is supposed to be overspread. These gratuitous
circumstances, grafted on each other, vanished in presence of Herschel's
observations. After having put himself on his guard against the effects
of diffraction, that is to say, against the scattering that light
undergoes when it passes the terminal angles of bodies, the illustrious
astronomer proved, in 1786, that objects can be seen well defined by
means of pencils of light whose diameter does not equal five tenths of a
millimetre.

Herschel looked on the almost unanimous opinion of the double lens
eye-piece being preferable to the single lens eye-piece, as a very
injurious prejudice in science. For experience proved to him,
notwithstanding all theoretic deductions, that with equal magnifying
powers, in reflecting telescopes at least (and this restriction is of
some consequence), the images were brighter and better defined with
single than with double eye-pieces. On one occasion, this latter
eye-piece would not show him the bands of Saturn, whilst by the aid of a
single lens they were perfectly visible. Herschel said: "The double
eye-piece must be left to amateurs and to those who, for some particular
object, require a large field of vision." (_Philosophical Transactions,
1782, pages 94 and 95._)

It is not only relative to the comparative merit of single or double
eye-pieces that Herschel differs from the general opinions of opticians;
he thinks, moreover, that he has proved by decisive experiments, that
concave eye-pieces (like that used by Galileo) surpass the convex
eye-piece by a great deal, both as regards clearness and definition.

Herschel assigns the date of 1776 to the experiments which he made to
decide this question. (_Philosophical Transactions_, year 1815, p. 297.)
Plano-concave and double concave lenses produced similar effects. In
what did these lenses differ from the double convex lenses? In one
particular only: the latter received the rays reflected by the large
mirror of the telescope, a