214 Mr Sang on an Improvement in the 



employed in crystallographic researches, is the reflecting gonio- 

 meter contrived by Wollaston ; but that instrument, in its ordi- 

 nary form, is of troublesome and uncertain application. It is, 

 in fact, an incomplete instrument, inasmuch as it does not con- 

 tain, in its own construction, all the elements from which the de- 

 terminations are to be made. Before it can be applied to the 

 measurement of the inclination of two faces of a crystal, a pair 

 of parallel lines must be traced at a considerable distance from 

 the table or stand on which the instrument is placed, and exact- 

 ly at equal distances from the axis of its motion. The axis has 

 also to be rendered parallel to these lines ; and all this is prepa- 

 ratory to the adjustment of the crystal. The goniometer in 

 this form is much inferior to the spindle of the turning lathe. 

 It is, for accurate purposes, as completely a fixture as the lathe, 

 and at the same time wants its steadiness and dimensions. I 

 long employed the head of my lathe as a goniometer, and, ha- 

 ving obtained a graduation to single minutes from Mr Adie, I 

 continue to employ it whenever the objects are of considerable 

 dimensions. By measuring the height of the reflecting surface 

 above the centre, as given by the tool of the slide-rest, the argu- 

 ment of the parallax is readily obtained, and thus nothing more 

 is left to be desired than portability. By a very simple con- 

 trivance, even the repetition of the angle can be obtained, and 



quantity, and the reflected image of A will be displaced from its apparent co- 

 incidence iii direction with B, by twice the angle through which the reflect- 

 ing surface of the crystal has been shifted. The result is, that the slightest 

 unsteadiness of the hand or the instrument produces some uncertainty in 

 making the coincidence. By using a permanent reflecting surface attached to 

 the instrument itself, to afford a second image of A to answer the end of the 

 object B, any displacement or tremor of the instrument affecting both sur- 

 faces equally, does not impair the accuracy of the adjustment, so that the in- 

 strument in its improved form may be as accurately used in the hand (like 

 the reflecting circle or sextant) as when clamped to the firmest table. If 

 the distance of the object A be considerable, the adjustment amounts simply 

 to making the reflecting plane of the crystal parallel to the permanent reflect- 

 ing plane secured to the instrument. 



Your committee are persuaded, from actual experiment, that this addition 

 to the reflecting goniometer, although so simple, is one of great practical con- 

 sequence; and the reporter desires to add, from his personal knowledge, that, 

 although only now presented to the Society, Mr Sang has employed this prin- 

 ciple in practice for several years past. 



JAMES D. FORBES (Reporter) 

 MthMay 1836. 



