Intelligence and Miscellaneous Articles. 237 



the two glasses must be increased to force the ring of the order n 

 to come and take the place of the central spot, and that in the dif- 

 ferent prismatic colours. According to the theory this quantity is 

 equal to n times half the wave-length of the light employed. The 

 apparatus used by me, in the laboratory of instruction of the Faculty, 

 to verify this point of the theory, was constructed with much care 

 by M. Laurent-Duboscq. 



* The rings are formed as usual, between a lens and a plane. The 

 lens is fixed, or has only revocable movements ; the plane moves 

 with regularity under the action of a good micrometric screw, the 

 channel of which was ordered equal to half a millimetre. Now, if 

 some parts quite at the extremities of this screw be excepted, what- 

 ever the part which is found in the nut, we may assure ourselves 

 at first that the angular course necessary for causing sixty consecu- 

 tive rings to enter into or issue from the central spot is constant. 



This first point verifies the exactness of the screw, and that, as 

 is evident, by a process simple, rapid, and, it seems to me, capable 

 of being applied in many cases. 



Secondly, I have observed that this angular course corresponding 

 to sixty rings is 12°-85. 



We have, then, if the channel is quite | a millhn., 



\ _ _ 12-85 

 60 



whence we deduce 



\=O000594 millhn. 



The fight employed was that of gas salted ; and it is pretty ge- 

 nerally admitted that the wave-length of the rays in the vicinity of 

 the line D is 0-000589. 



It is easy to perform analogous experiments by illuminating the 

 glasses with sensibly homogeneous rays from the different regions 

 of a good spectrum ; and I have ascertained that the spectrum of a 

 Drummond lamp is quite intense enough for this purpose. 



The number of rings which pass with an entire turn of the screw 

 is 1683 when the operation takes place with the light in the vicinity 

 of the line D. JSTow, as the button with which the action is per- 

 formed is only 4 centims. in diameter, it is clear that the hand must 

 be moved very slowly in order that several rings at once may not 

 be made to enter the central spot. When we wish to limit our- 

 selves to observing the passage of fifteen rings only, it is preferable 

 to conduct the principal motor screw with a tangent micrometric 

 screw itself. In my apparatus, an entire turn of the tangent screw 

 corresponds to only two and a half rings ; consequently one can 

 easily divide into more than 100 equal parts the movement of the 

 ring which answers to a displacement of 0-000592 in the movable 

 glass. — Comptes JRendus de VAcad. cles Sciences, January 26, 1874. 



60 o =0*5 o^m millhn., 



ON EXTRAORDINARY DEVELOPMENTS OF LIGHT IN THE GRINDING 

 OF HARD STONES. BY DR. JACOB NOGGERATH. 



It is known that a number of minerals and stones become phos- 

 phorescent by friction, especially on being rubbed against each other 



