TRANSACTIONS OF THE SECTIONS. 19 
Pit is only by such cooperation of those engaged in different depart- 
_ ments of science that inquiries like the present can be successfully 
_ carried on, and the author is anxious to obtain specimens of any trans- 
_ parent media which are capable of prismatic examination, and espe- 
_ cially such as are of high dispersive power. 
__ Meanwhile he has been engaged in the comparison of observation 
_ and theory; especially among the more highly dispersive of those 
_ media which he has examined. He has performed the calculations by 
_ the method of Sir W. R. Hamilton, and has found that for those media 
whose dispersion is not very great, the coincidences are sufficiently 
close; but on proceeding to the more highly dispersive bodies, espe- 
cially oil of cassia, tle discrepancies increase ; and, moreover, preserve 
a certain regularity of character which shows that they are not mere 
errors of observation. This would seem to warrant the expectation, 
that a further development of the formula might still give successful 
_ results. These investigations have been communicated to the Royal 
_ Society, and have now appeared in the Transactions. 
_ Since the period of this communication, however, the able and 
_ profound Memoir of Mr. Kelland appeared in the Cambridge Trans- 
actions. ‘This gentleman’s theory is, in some measure, a simplification 
of Cauchy’s; the resulting fermula for the dispersion, though sub- 
_ stantially the same, is developed in a different form, and readily capable 
of being applied to numerical computation. In some correspondence 
_ with Mr. Kelland, that gentleman favoured the author with a compu- 
tation for the case of oil of cassia, in which the greatest discrepancies 
existed. By this method those discrepancies have been made entirely 
to disappear; and thus the most ewtreme case at present known is 
brought under the dominion of the formula of dispersion. It is also 
to be observed that Mr. Kelland’s series is not rapidly converging ; the 
neglected terms therefore may, if taken into account, give a still more 
accurate result. These results will appear in the Philosophical Trans- 
actions. 
It will now, therefore, become of yet more extreme interest to find 
some means of obtaining data for the more highly dispersive sub- 
_ stances, such as chromate of lead, realgar, sulphur, &c. 
With regard to the theoretical computation, it must be owned, after 
all, that it is not altogether satisfactory in its nature, as it assumes three 
- indices from observation, and thence determines the others, which is 
in fact a process of interpolation, and does not explain the character, 
_ of the dispersion as referring to those three indices. Whether the 
_ theory can be improved in this respect becomes an important topic of 
_ inquiry. 
_ But the whole subject has now been most ably examined by 
Bifeasor Lloyd, whose papers have been communicated to the Royal 
sh Academy, and include several highly curious and important 
_ theoretical conclusions relating to the whole subject of the propaga» 
tion of light in uncrystallized media. 
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