Intelligence and Miscellaneous Articles. 157 



"to 



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 b^ 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 win become lessened to an extent corresponding to 2 «, which 

 must be added to this new 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 «. 



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 difi"erent instants, so that one will 

 be somewhat later than the other, the reflected images will no longer 

 jjreserve 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. 



