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 the angle through which the radius has moved, which angle is measured by the portion of the limb it has swept over.

The limb is graduated, so that the zero point answers to that position of the radius, which makes the image of an object cover it, and half degrees of the arc of the instrument are counted as degrees; so that the angle between two objects is found at once, by observing the number of degrees marked on the limb, when they are brought into apparent contact.

The plain mirrors on this instrument are sometimes supplied with advantage by two prisms, which are placed so as to produce a total reflexion of the light. Fig. 160, represents a sextant invented by Professor Amici of Modena, which is described in the Correspondance Astronomique du Baron de Zach.

154. The curious images seen in this amusing toy, are produced by two glass mirrors placed so as to touch along one edge, and to form, by their inclination, an angle which is some submultiple of two right angles. The eye is placed as in Fig. 161, and the object is generally a collection of bright glass beads and fragments, inclosed between two disks of glass, at the other end of the instrument. The glass mirrors are smoked, or covered with some rough black substance on the back, to prevent reflexion at the second surface.

The formation of the images will be easily understood from Fig. 162, where $AO$, $BO$, are the mirrors, inclined at an angle of 60 degrees. The figure $a$, placed between them, is reflected at each so as to occupy, in inverted positions, the two sections $AOF$, $BOC$. These images are again reflected at $BO$, $AO$, and from other images in $COD$, $FOE$, which, in this case, conspire to form by another pair of reflexions another image in $EOD$ which completes the circle.

155. This pretty little instrument invented by Dr. Wollaston, and which is of great assistance in drawing from nature, consists mainly of a glass prism, the section of which, represented in Fig. 164, is a trapezium, having one right angle ($B$), and an angle of 135°