Its construction is shown in fig.7.ABC is a totally reflecting prism of glass.

The rays from the object-glass fall on the face AB, are totally reflected on the face BC, and emerge through the face AC.

In using this eye-piece, it must be remembered that it lengthens the sliding eye-tube, which must therefore be thrust further in, or the object will not be seen in focus.

There is an arrangement by which the change of direction is made to take place between the two glasses of the eye-piece.

With this arrangement (known as the _diagonal eye-piece_) no adjustment of the eye-tube is required.