. The Family tutor . Fig. 42. principle of this instrument may be un-derstood by reference to fig. 42, which * From the Greek gonia, an angle, and metron,a measure. represents a crystal (c), whose angle (a h c)is required. The eye at p, looking at theface {h c) of the crystal, observes a reflectedimage of M, in the direction r n. Thecrystal may now be so turned that thesame image is seen reflected in the nextface (b a), and in the same direction (p n).To effect this the crystal must be turneduntil a h has the present position of h angle d b c measures, therefore, thenumber of degrees thr
. The Family tutor . Fig. 42. principle of this instrument may be un-derstood by reference to fig. 42, which * From the Greek gonia, an angle, and metron,a measure. represents a crystal (c), whose angle (a h c)is required. The eye at p, looking at theface {h c) of the crystal, observes a reflectedimage of M, in the direction r n. Thecrystal may now be so turned that thesame image is seen reflected in the nextface (b a), and in the same direction (p n).To effect this the crystal must be turneduntil a h has the present position of h angle d b c measures, therefore, thenumber of degrees through which thecrystal must be turned. But d b c sub-tracted from 180 equals the requiredangle of the crystal a b c ; consequently,the crystal passes throixgh a number ofdegrees, which, subtracted from 180°,gives the required angle. When ttie crys-tal is attached to a graduated circle, whichshould move with it, we have the gonio-meter of Wollaston. In figure 43 there. Fig. 43. is shown such a circle of brass, a, gra-duated to half degrees, and hung by theaxis b, on which it moves with greatsteadiness. This axis is perforated fromend to end for the passage of a closelyfitting rod or central axis, on one end ofwhich is the bent joint (d), carrying thecrystal (/). By the head c, and thearrangement at d, the crystal is adjustedwithout moving the graduated wheel; andwhen this is accomplished in such a man-ner that the eye of the observer placedover the crystal, as at p (fig. 43), can seea clear image of a line on the wall (m), ora window-bar in each face successively,then the graduated wheel (which stands,when at rest, at zero of the vernier (e) ismade to revolve), and it the crystal, 192 CHEMISTRY. until the mark or window-bar is distinctlyseen in the second face. The mimber ofdegrees and parts of a degree which cor-respond to the angle required, are thusobtained directly by the movement of thewheel, which was beforehand placed with180° opposite to th
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Keywords: ., bookcentury1800, bookdecade1850, bookpublisherlondo, bookyear1851
