Page:The wonders of optics (1869).djvu/255

Rh other drawing. It was invented by Dr. Wollaston, in 1804, and consists of a little four-sided prism, of which fig. 71 is a section.



The angle at is a right angle; the angle  measures 67-1/2°, the angle  135°, and the angle  is, of course, equal to. It is mounted on a sliding foot, so that it may be raised or lowered at will, or turned in a horizontal direction. The path of the rays in this case is easy to follow, the object to be copied being placed at, and the eye at. On looking downwards the image of the object to be drawn is seen on the paper; and if the eye is placed so that the edge of the prism will just cut the pupil in two, the paper and pencil will be seen at the same time. It will be seen from the diagram, that the rays proceeding from strike on the surface  at right angles, and, being then reflected from, pass upwards again to point. The direction of the rays is in reality a little more complicated than this. In the case of distant objects it is impossible to see both the object and the pencil at the same time; a lens is sometimes introduced at to modify this defect. The original instrument has also been modified by the intro