. Astronomy for students and general readers . sts of the arc of a divided circle usually 60° in extent,whence the name. This arc is in fact divided into 120 equal parts,each marked as a degree, and these are again divided into smallerspaces, so that by means of the vernier at the end of the index-armJfir 5 an arc of 10 (usually) may be read. The M 8 carries the index-glass M, which is a silveredplane mirror set perpendicular to the plane of the divided arc. The IHE SEXTANT. 93 horisoTi-glass m is also a plane mirror fixed perpendicular to theplane of the divided circle. This last g
. Astronomy for students and general readers . sts of the arc of a divided circle usually 60° in extent,whence the name. This arc is in fact divided into 120 equal parts,each marked as a degree, and these are again divided into smallerspaces, so that by means of the vernier at the end of the index-armJfir 5 an arc of 10 (usually) may be read. The M 8 carries the index-glass M, which is a silveredplane mirror set perpendicular to the plane of the divided arc. The IHE SEXTANT. 93 horisoTi-glass m is also a plane mirror fixed perpendicular to theplane of the divided circle. This last glass is fixed in position, while the first revolves withthe index-arm. The horizon-glass is divided into two parts, ofwhich the lower one is silvered, the upper half being transparent.^ is a telescope of low power pointed toward the it any object to which it is directed can be seen through the un-sil^ered half of the horizon-glass. Any other object in the sameplane can be brought into the same field by rotating the index-arm. Fig. 40.—the sextant. (and the index-glass with it), so that a beam of light from thissecond object shall strike the index-glass at the proper angle, thereto be reflected to the horizon-glass, and again reflected down thetelescope M Thus the images of any two objects in the plane ofthe sextant may be brought together in the telescope by viewingone directly, and the other by reflection. The principle upon which the sextant depends is the following,which is proved in optical works. The angle tetween the first andthe last direction of a ray which has suffered two reflections in the same 94 ASTRONOMY. plane is equal to twice the angle which the two reflecting sutfaees makewith each other. In the figure 5 ^ is the ray incident upon A, and this ray is byreflection brought to the direction B E. The theorem declaresthat the angle B E S is equal to twice D C B, or twice the angle of
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