6._] The course of a slightly oblique pencil through either eye-piece is exhibited in the figures.

The lenses are usually plano-convex, the convexities being turned towards the object-glass in the negative eye-piece, and towards each other in the positive eye-piece.

Coddington has shown, however, that the best forms for the lenses of the negative eye-piece are those shown in fig.

5.
The negative eye-piece, being achromatic, is commonly employed in all observations requiring distinct vision only.

But as it is clearly unfit for observations requiring micrometrical measurement, or reference to fixed lines at the focus of the object-glass, the positive eye-piece is used for these purposes.
For observing objects at great elevations the diagonal eye-tube is often convenient.