18 SEVENTH REPORT—1837. 
Meanwhile, as to the theory, that part of it which refers to the 
mode of aggregation of the particles of bodies, is necessarily, as yet, 
hypothetical; and we may therefore still consider as worthy of atten- 
tion any other principles which may be suggested. 
The point on which it is probable any theory must essentially turn 
is that of a retardation of some part of the light within the medium, 
and its emerging along with the direct ray in a state of interference. 
A ray which enters a medium perpendicularly, though not refracted 
as to direction, is yet retarded in proportion to the refractive index for 
that ray and that medium. Another ray coinciding with it, and 
having a refractive index slightly different, will be wnequally retarded ; 
and however small the difference may be, yet in a considerable thick- 
ness it may amount to a discordance between the two rays when they 
emerge; and if their wave lengths differ only by a very small quan- 
tity, they may so interfere as to produce a sensible destruction. 
If two media are compounded together which have the same re- 
fractive index for one ray, and different indices for a ray whose wave 
length differs very little from the first, that which retards it most will 
prevail, and the two rays may interfere and produce darkness from 
this cause. 
But the recent theoretical researches of Professor Lloyd, communi- 
cated to the Royal Irish Academy, seem to promise an explanation of 
the absorption, and with views somewhat different, connected with his 
profound investigations on the propagation of light, and dependent 
on the mathematical form which the expressions assume under certain 
conditions. This was also, to a certain extent, a consequence from 
some of the analytical investigations of Mr. Tovey on the dispersion. 
On the Dispersion of Light. By Prorressor PoweEtt. 
The object of this communication is to state the progress of the 
inquiry into the subject of dispersion since the last meeting of the 
Association. On that occasion the author laid before the physical 
section the results of his observations for determining the refractive 
indices of the standard rays for twenty-eight media. These have been 
since published, with some preliminary remarks, as one of the series 
of Memoirs of the Oxford Ashmolean Society. They are to be con- 
sidered only as first approximations, and it would be very desirable to 
have many of them carefully repeated, as well as to extend the inquiry 
to other bodies. The author regrets that he has been unable, from 
particular circumstances, to carry on these researches during the past 
summer, but intends to take the first opportunity of resuming them. 
In particular, he was kindly favoured by Mr. Brooke with a specimen 
of'some erystals of chromate of lead for examination, and accordingly 
put them into the hands of Mr. Dollond, who warmly entered into his 
views, and after many vain endeavours to give them a prismatic form, 
has at length succeeded in forming a very minute prism which is under 
trial. 
