_Ramsden’s eye-piece._--There is another modification of lenses, known by the name of the _Positive_, or Ramsden’s eye-piece, which is much used in Transit instruments, and telescopes which are furnished with micrometers, and which affords equally good vision as the other eye-piece. In this construction the lenses are plano-convex, and nearly of the same focus, but are placed at a distance from each other less than the focal distance of the glass next the eye, so that the image of the object viewed is beyond both the lenses, when measuring from the eye. The flat faces of the two lenses are turned into contrary directions in this eye-piece--one facing the object-glass, and the other the eye of the observer; and as the image formed at the focus of the object-glass, lies parallel to the flat face of the contiguous lens, every part of the field of view is distinct at the same adjustment, or, as opticians say, there is a _flat field_, which, without a diaphragm, prevents distortion of the object. This eye-piece is represented in fig. 74, where AB and CD are two plano-convex lenses, with their convex sides inwards. They have nearly the same focal length, and are placed at a distance from each other, equal to about two thirds of the focal length of either. The focal length of an equivalent single lens is equal to three fourths the focal length of either lens, supposing them to have equal focal distances. This eye-piece is generally applied, when wires of spider’s lines are used in the common focus; as the piece containing the lenses can be taken out without disturbing the lines, and is adjustable for distinct vision; and whatever may be the measure of any object given by the wire micrometer, at the solar focus, it is not altered by a change of the magnifying power, when a second eye-piece of this construction is substituted. [Illustration: _figure 74._] [Illustration: _figure 75._] [Illustration: _figure 76._]