370 Count de Bournon on BardigUone, 



crystal, and which probably deceived the Abbe Haiiy. On looking 

 at the crystals of this substance, through their terminal faces oppo- 

 site to the light, lines are observed in the interior of several, perfectly 

 distinct, and in the direction of the two diagonals of these terminal 

 faces. The intersection of these lines, instead of taking place perpen- 

 dicularly, so as to be at right angles, and thus forming squares, as in 

 figs. 3 and '4, appears to be made obliquely, fig. 5, so as to form 

 rhombs, as shown at fig. 6. If, in order to find the measure of the 

 angles of these rhombs, different angles be placed on them till their 

 sides apparently coincide, an angle of 100*, or nearly so, appears to 

 agree very well with the obtuse angles, so that the rhombs have ap- 

 parently 1 00° and 80° for the measures of their angles. But then if 

 a natural, or an artificial fracture, the latter of which is ver^^ difficult, 

 be made according to the natural diagonal joints of the primitive 

 crystal of this substance, the plane produced should make with 

 the adjacent planes of the prism on one side an angle of 140° 3', 

 and on the other an angle of 129° 57'; which never is the case, for 

 these two angles are always very exactly 135", being what the same 

 section must produce on the supposition of the sides of the terminal 

 faces being equal. Yet if we compare the direction of this face, with 

 the lines traced in the interior of the crystal, according to the natural 

 joints, it appears to be perfectly parallel with that of these lines. 



Such are the reasons which determined me to consider the base 

 of its primitive rectangular tetrahedral prism as a square : yet, as I 

 have observed above, there is in this substance, on account of the 

 difference between the appearance of the angles formed by the 

 meeting of the interior lines, indicating the natural joints ; and the 

 correspondent ones formed by the planes parallel to these joints, 

 something very peculiar, which would seem owing to some illusion 

 dependent on refraction, for which I cannot account. 



