Intelligence and Miscellaneous Articles. 157 
montite may therefore be taken as a right prism with square bases, 
in which the relation between the sides of the base and the height 
is nearly as the numbers 23 and 10, and then the faces of the pyra- 
mid have as a crystallographical sign. The crystals cleave readily 
parallel to the lateral faces of the primary form, but more easily 
parallel to one of the faces than the other, and this greater facility 
corresponds with the pearly lustre peculiar to them. There are also 
some indications of cleavage, parallel to the diagonal planes of the 
primary form/ the crystallographical sign of which is g^. The colour 
of the crystals is whitish yellow ; they are translucent ; their hard- 
ness is greater than that of haydenite, and is almost equal to that of 
phosphate of lime. 
The crystals of beaumontite and haydenite form a crystalline 
layer, the brilliant portions of which belong to the first-mentioned 
substance, and the parts covered with brownish hydrate of iron to the 
second. This layer covers a granular rock composed principally of 
grains of quartz and haydenite. The other face of the specimen 
is covered with small flat elongated prisms of green amphibole. — 
Ibid, 
AN ACCOUNT OF THE EXPERIMENT PROPOSED BY M. ARAGO AS 
A TEST OF THE ACCURACY OF THE UNDULATORY HYPOTHE- 
SIS OF LIGHT. * 
M. Arago proposes to avail himself of Prof. Wheatstone’s revol- 
ving mirror, used by that gentleman in his researches on the velocity 
of electricity whilst traversing good conductors, in determining ex- 
perimentally the accuracy of one or other of the present hypotheses 
of light. The principle of the proposed experiment is readily un- 
derstood. A ray of light incident on the surface of a plane mirror 
is reflected in the ordinary manner, the reflected and incident beams 
forming equal angles with a line perpendicular to the point of inci- 
dence of the ray. If the mirror be supposed to revolve around this 
point to an extent expressed by the quantity a, and to augment by 
this quantity the original angle of incidence, the former angle of re- 
flexion will become lessened to an extent corresponding to 2 a, which 
must be added to this nev/ angle to render it equal to the first. 
Consequently, if the incident ray remain the same, an angular move- 
ment of the mirror of a will produce an angular motion of the re- 
flected ray equal to 2 a. 
If then two perfectly parallel rays be incident in the same vertical 
line on a mirror revolving round the point of incidence, their paral- 
lelism will be preserved after reflexion, providing they both impinge 
upon the mirror at the same instant of time, and two luminous points 
situated exactly vertically over each other will be seen ; but if the 
rays impinge upon the mirror at different instants, so that one will 
be somewhat later than the other, the reflected images will no longer 
preserve their original position in the same vertical line — one ap- 
pearing to the right or left of the other. 
* The Editors are obliged to Dr, Golding Bird for this account. 
