SPHKRICAT. ELEMENTS OF CRYSTAL*. 
207 
' be a regular rhomboid ; but the measures of its angles will be 
different from those of the former, and will be more or less 
obtuse, according to the degree of oblateness of the primitive 
I spheroid. 
It is, at least, possible, that carbonate of lime and other sub- /^g possibly in 
I stances, of which the forms are derived from regular rhom- carbonate of 
... Iliac; 
i boids as their primitive form, may, in fact, consist of oblate 
• spheroids as elementary particles. 
It deserves to be remarked, that the conjecture to which and conjertur" 
’ we are thus led by a natural transition, from consideration of 
the moiit simple form of crystals, w.is long since entertained by 
Huyghens*, when treating of the oblique refraction of Iceland 
spar, which he so skilfully analysed. The peculiar law ob- 
•servable in the refraction of light by that crystal, he found 
I might be explained on a supposition of spheroidical undula- 
Itions propagated tlirough the substance of the spar, and these, 
Ihe thought, might, perhaps, be owing to a spheroidical form of 
its particles, to which the disposition to split into the rhomboidal 
tform might also be ascribed. 
By some oversight, however, the proportion of the axes of Numerical di. 
•such an elementary spheroid is erroneously stated to be 1 jq 
• 8 ; but this is probably an error of the press, instead of 1 to 
2,8, for I find the proportion to be nearly 1 to 2'87. In fig. 
15, F is the apex of a tetrahedron cut from an acute rhomboid 
(similar to fluor spar, and the sections of two spheres are re- 
presented round the centres F and C. I is the apex of a cor- 
responding portion cut from the summit of a rhomboid of 
IJceland spar, as composed of spheroids having the same dia- 
' meter as the spheres. In the former, the inclination FCT of 
[the edge of the tetrahedron to its base is 54“ 44' ; in the latter, 
the inclination ICT is 2d” 15'; and the altitudes FT, IT, are as 
the tangents of these angles 1414 to 4f)3 : :2 87 ^ I» which 
also expresses the ratio of the axis of the sphere to that of 
the spheroid, or the proportional diameters of the generating 
ellipse. 
• Hii}ghenii Op. Rdiq. Tom, 1. Tract dc Liimine, p. 70. 
Hex. 
