of Chromatic Dispersion. 369 
the refrangibility of the wave by a like definite amount, as a 
consequence of the curtailment of the amplitude. This curtail- 
ment being absolutely the same for all the waves, will produce 
the greatest rateable effect on those undulations whose vibrations 
have the least amplitude, namely those at the extremes of the 
spectrum. Hence these will have their refraction increased from 
the operation of this cause in a higher proportion than will the 
central waves, and so produce a displacement of the fixed lines 
from their normal positions in the manner observed. 
Or again, the slight alterations in the rapidity of the indivi- 
dual vibrations of the ethereal particles comprised within the 
wave-length, and which manifest themselves by the displacement 
of the fixed lines, may be due to a sympathetic action between 
the vibrations of the ponderable atoms of the medium and 
those of the «ethereal particles, somewhat resembling the sym- 
pathy of vibrating pendulums; and in virtue of this sympathy 
some of the «ethereal vibrations may be slightly increased, and 
others slightly diminished in their rate of rapidity beyond what 
they would otherwise be. These views, however, are merely 
thrown out as hints for consideration. 
While one of the objects with which the foregoing investiga- 
tion is submitted to the British Association is to call the atten- 
tion of its members to the curious laws governing the displace- 
ment of the fixed lines, and their use in detecting errors of ob- 
servation, and to bring under their notice the advantages of the 
exponential law as furnishing a method of calculating accurate 
indices of refraction from tolerably correct observations, the chief 
purpose is to awaken those who take a lead in the proceedings 
of the Association to the necessity which exists for a careful repe- 
tition of those observations that have been found most deficient 
in accuracy, and for an extension of the observations generally. 
It would be particularly interesting to accumulate observations 
at different temperatures, so as further to illustrate the law of 
the variation of the index of elasticity, viewed in relation to the 
contractions and expansions of the medium under the influence 
of changes of temperature, and also in relation to the capacity 
of the medium for heat. It is not improbable that, out of an 
experimental investigation carefully conducted in this direction, 
some highly interesting and important results might arise. 
It is only under the direction and auspices of such a body as 
the British Association that there is any likelihood of an experi- 
mental investigation of such a nature being undertaken; and 
it is hoped that, when its importance to the science of physical 
optics shall have been duly weighed by them, they will not fail 
to place the matter in the hands of competent and careful ob- 
servers, so that an enlarged store of trustworthy experimental 
data may be obtained. 
