lanets offers unquestionable advantages. The orb itself is a minute
star-like point in the telescope, and the measures are made from it to
the stars which are seen near it. A few words will, perhaps, be
necessary at this place as to the nature of the observations referred
to. When we speak of the measures from the planet to the star, we do not
refer to what would be perhaps the most ordinary acceptation of the
expression. We do _not_ mean the actual measurement of the number of
miles in a straight line between the planet and the star. This element,
even if attainable, could only be the result of a protracted series of
observations of a nature which will be explained later on when we come
to speak of the distances of the stars. The measures now referred to are
of a more simple character; they are merely to ascertain the apparent
distance of the objects expressed in angular measure. This angular
measurement is of a wholly different character from the linear
measurement, and the two methods may, indeed, lead to results that would
at first seem paradoxical.

We may take, as an illustration, the case of the group of stars forming
the Pleiades, and those which form the Great Bear. The latter is a large
group, the former is a small one. But why do we think the words large
and small rightly applied here? Each pair of stars of the Great Bear
makes a large angle with the eye. Each pair of stars in the Pleiades
makes a small angle, and it is these angles which are the direct object
of astronomical measurement. We speak of the distance of two stars,
meaning thereby the angle which is bounded by the two lines from the eye
to the two stars. This is what our instruments are able to measure, and
it is to be observed that no reference to linear magnitude is implied.
Indeed, if we are to mention actual dimensions, it is quite possible,
for anything we can tell, that the Pleiades may form a much larger group
than the Great Bear, and that the apparent superiority of the latter is
merely due to its being closer to us. The most accurate of these angular
measures are obtained when two stars, or two star-like points, are so
close together as to enable them to be included in one field of view of
the telescope. There are special forms of apparatus which enable the
astronomer in this case to give to his observations a precision
unattainable in the measurement of objects less definitely marked, or at
a greater apparent distance. The determination o