It may appear a curious circumstance that any eye-piece which is good with a short telescope, is also good with a long one, but that the reverse is not true; for it is found to be more difficult to make a good eye-piece for a short than for a long focal distance of the object-glass. Celestial eye-pieces are sometimes constructed so as to produce _variable powers_. This is effected by giving a motion to the lens next the eye, so as to remove it nearer to or farther from the field lens; for at every different distance at which it is placed from the other lens, the magnifying power will either be increased or diminished. The greatest power is when the two lenses are nearly in contact, and the power diminishes in proportion to the distance at which the glass next the eye is removed from the other. The scale of distance, however, between the two lenses, cannot be greater than the focal distance of the field, or inner glass; for if it were, the lenses would no longer form an eye-piece, but would be changed into an inverting opera-glass. For effecting the purpose now stated, the eye-glass is fixed in a tube which slides upon an interior tube on which is marked a scale of distances, corresponding to certain magnifying powers; and, in this way an eye-piece may be made to magnify about double the number of times, when the lenses are in one position than when they are in another--as, for example, all the powers from 36 to 72 times may be thus applied, merely by regulating the distance between the two lenses. When the glasses are varied in this manner the eye-piece becomes sometimes a _positive_ eye-piece, like Ramsden’s, and sometimes a negative one like that of Huygens.