M. C. Lea—Action of the less refrangible rays of Light, ete. 269 
a tuning fork is brought near the mouth of the resonator; on 
revolving the perforated disc, two additional, or secondary, 
sounds appear; one slightly above, the other slightly below 
the pitch of the fork. An increased velocity of rotation causes 
the two secondary sounds to diverge yet farther from the note 
of the beating fork, until the velocity reached is so great that 
the two secondary sounds become separated from each other 
by a major sixth, while, at the same moment, a resultant sound 
appears, formed by the union of the sound of the fork with the 
upper and the lower of the secondary sounds. This resultant 
is the second octave below the note given by the fork. On 
further increasing the velocity of the disc, the two secondary 
sounds and the resultant disappear, and the ear has alone the 
sensation of the simple sound produced by the beats of the 
fork ; which, at this stage of the experiment, blend into a 
smooth continuous sensation. These successive and gradual 
changes, as they happen with a UT, fork, we have indicated 
in steps of semi-tones in the appended musical notation. The 
In conclusion, 1 request my readers to transfer the comma 
from after Déssonanz to after continuirliche, in the quotation 
from Helmholtz at the beginning of my paper (vol. viil, p. 241). 
Art. XXIX.—On the Action of the Less Refrangible Rays of 
Light on Silver Iodide und Bromide; by M. Carsy Lua, 
Philadelphia. 
Ir will be the object of the present investigation to show: 
Ist. That silver iodide and bromide are sensitive to all the 
colored rays of the spectrum. 
2d. That silver iodide is to all the less refrangible rays more 
Sensitive than silver bromide. 
3d. That the theory of M. E. Becquerel as to existence of 
“exciting rays” and “continuing rays” is not supported by a 
careful examination of the phenomena in question. 
The first of these positions differs from those generally ac- 
cepted in extending eae to the less refrangible end of the 
