THE 



EDINBURGH NEW 

 PHILOSOPHICAL JOURNAL. 



Account of an Improvement in the Construction (*f Wollastorfs 

 Goniometer. By EDWARD SANG, Esq., F. R. S. E., Vice- 

 Pres. Soc. Arts, Teacher of Mathematics, &c. Edinburgh. * 



THE science of crystallography has now become, on account 

 of its connection with the new subject of polarized light, a most 

 important branch of stereometry. Almost the only instrument 



* Read before the Society of Arts for Scotland, 13th April 1836, the 

 Society's Honorary Silver Medal awarded 7th December 1836*. 



Report on Mr gang's Improved Form of Wollaston's Goniometer. 



Your committee have examined carefully Mr Sang's addition to the go- 

 niometer, which consists of a plane mirror capable of being adjusted, so that 

 the plane of reflection shall be perpendicular to the axis of the instrument. 



Instead of bringing the image of an object (such as the bar of a window) 

 reflected from the surface of the crystal whose angle is to be measured, to 

 coincide in direction with a second object (such as a window bar parallel to the 

 first), Mr Sang proposes to employ the reflection of a single object from a 

 plane mirror attached to the instrument. 



This modification, though very simple, affords great facility of practical ap- 

 plication. In using the instrument in its usual form, the image of the first 

 object (A), reflected from the crystal, can only be brought to coincide in di- 

 rection with the second object (B), seen by direct vision, whilst the instru- 

 ment remains perfectly at rest : and this however distant either object. Alter 

 the position of the goniometer, and the crystal attached to it by the smallest 

 VOL. XXII. NO. XLIV. APRIL 1837. P 



