Measurement and Cleavage of certain Primitive Crystal*. 249 



brilliant for the use of the reflecting goniometer ; that instrument 

 therefore, in regard to this substance, has been used only to measure 

 the angles by means of the reflections of the natural planes of the 

 crystals ; but as the hyacinth of France is always too much water- 

 worn to present those well defined reflections which alone can be 

 relied on, and which frequently occur on the smallest and most 

 transparent crystals of the jargoon, I first depended on the latter, 

 but have since been enabled by the examination of a large quantity 

 of hyacinths, to find some crystals which, though dull, afford the 

 same results. 



These results differ from those obtained by Haiiy, no less than 

 one degree and a half, which caused me to measure over again the 

 whole number of crystals, but without discovering any error. The 

 incidence of P on P is given both in the Traite and Tableau Com- 

 paratif as 82° 50', leaving of course the incidence, of P on the 

 opposed plane over the summit 97° 10', as the complement. But 

 as the crystals of jargoon in my possession, rarely exhibit both 

 pyramids, and never sufficiently brilliant to be relied on, I have 

 been compelled to depend on measurements obtained on the plane 

 P and the opposed plane over the apex. Clear reflections agree in 

 five instances in affording 95° 40', in two or three 95° 35', and in 

 one instance 95° 30' ; while the only incidence of P on P is 84° 15', 

 being five minutes short of what I conceive to be the true value of 

 the angle, viz. 84° 20'. Two fragments exhibiting planes parallel 

 with the faces of the primitive octohedron, but not sufficiently 

 bright for the use of the reflecting goniometer, afford by that in 

 common use, an angle of about 95° 40' ; two others of about 

 84° 20'. 



I have now stated the reasons which induce me to assume the 

 true measurement of P on P to be one degree and a half greater 



