REFLECTIVE GONIOMETER. 67 



the window, with its upper edge nearly on a level with the axis of 

 the instrument. Let a horizontal line be drawn with a knife across 

 the slate near its upper edge and parallel (as in the case of the black 

 line on the wainscot,) with the bars of the window. Now let the 

 slate be fixed exactly at such a distance from the observer, when his 

 eye is at c, that he can most distinctly perceive the horizontal^rnark 

 upon the slate. Here let it be made stationary upon the table. In 

 this situation, the horizontal mark on the slate i to be substituted 

 for the black mark first described. But instead of the reflection of 

 a window bar, if the table should be situated near the window, it 

 will be better to substitute a piece of wood of half the diameter of 

 a window bar, placing it across half way between, and parallel 

 with, two contiguous bars, since a window bar will give at this 

 distance an image whose diameter is greater than is requisite, and 

 thus diminish the accuracy of the coincidence we obtain with the 

 line upon the slate. 



The instrument whose use has just been described has a remark- 

 able advantage over the common goniometer not only in the accu- 

 racy of its results, which in perfect crystals may be said to give the 

 value of angles within a minute of a degree, but in its application 

 to crystals of the smallest dimensions; it being estimated that it will 

 give the inclination of planes whose area is less than joVcr oir of 

 an inch. Indeed, the smaller the crystal is, in general, the more 

 eligible it becomes for the measurement of its angles by this go- 

 niometer, since in such crystals, inaccuracies are less liable to occur 

 from the curvature of the crystalline foces, than in those which are 

 of larger dimensions. 



Notwithstanding the superiority of the reflective over the com- 

 mon, goniometer in most cases, both instruments are indispensable 

 in the cabinet of every Mineralogist. Occasionally, crystals are 

 too large or are deficient in polish, for the use of the reflective 

 goniometer, in which case, the common one must be resort- 

 ed to. The common goniometer has an advantage over the re- 

 flective one in being a pocket instrument, as well as in the facility 

 of its application where the crystals are of a suitable size : the re- 

 flective goniometer, however, must be employed on all occasions 

 when the crystals will allow of it, where the object is to describe 

 accurately a new form of crystal, or to establish the system of crys- 

 tallization belonging to a species not yet generally known. 



