ch an eyepiece is
achromatic. It is therefore generally preferred for mere seeing
purposes. In the Ramsden eyepiece two plano-convex lenses are also used,
but they are of equal focal length, are placed at a distance apart equal
to two thirds of the focal length of either, and have their convex sides
facing one another. With such an eyepiece the image viewed is beyond the
farther or field lens instead of between the two lenses, and as this
fact renders it easier to adjust wires or lines for measuring purposes
in the focus of the eyepiece, the Ramsden construction is used when a
micrometer is to be employed. In order to ascertain the magnifying power
which an eyepiece gives when applied to a telescope it is necessary to
know the equivalent, or combined, focal length of the two lenses. Two
simple rules, easily remembered, supply the means of ascertaining this.
The equivalent focal length of a negative or Huygens eyepiece is equal
to half the focal length of the larger or field lens. The equivalent
focal length of a positive or Ramsden eyepiece is equal to three fourths
of the focal length of either of the lenses. Having ascertained the
equivalent focal length of the eyepiece, it is only necessary to divide
it into the focal length of the object glass (or mirror) in order to
know the magnifying power of your telescope when that particular
eyepiece is in use.

[Illustration: NEGATIVE EYEPIECE.]

[Illustration: POSITIVE EYEPIECE.]

A first-class object glass (or mirror) will bear a magnifying power of
one hundred to the inch of aperture when the air is in good
condition--that is, if you are looking at stars. If you are viewing the
moon, or a planet, better results will always be obtained with lower
powers--say fifty to the inch at the most. And under ordinary
atmospheric conditions a power of from fifty to seventy-five to the inch
is far better for stars than a higher power. With a five-inch telescope
that would mean from two hundred and fifty to three hundred and
seventy-five diameters, and such powers should only be applied for the
sake of separating very close double stars. As a general rule, the
lowest power that will distinctly show what you desire to see gives the
best results. The experienced observer never uses as high powers as the
beginner does. The number of eyepieces purchased with a telescope should
never be less than three--a very low power--say ten to the inch; a very
high power, seventy-five or one hundred