412 Unequal Refrangibility of Light on the undulatory Theoi-y. 



atoms of sulphur is the same in each of these minerals, or that 

 these metals can (?iw replace each other as isomorphous ingre- 

 dients ; so that all ground of comparison vanishes. 



I will conclude with noticing the obligation which the doc- 

 trine of isomoi'phous and plesiomorphous groups, as well as 

 other parts of mineralogy, owes to INIr. Brooke himself. Thus, 

 among other instances, (Ann, Phil. Aug. 1823,) he has re- 

 marked the near agreement in form, of sulphate of iron and 

 sulphate of cobalt : also (Ann. Phil. Dec. 1823) that of the sul- 

 phate of nickel and the sulphate of zinc ; and (Ann. Phil. Jan. 

 1824') the exact agreement of nitrate of lead and nitrate of 

 baryta. 

 Trinity College, Cambridge, Nov. 7, 1831. W. W. 



LI I. Ujiequal Refrangibility of Light on the undulatory 

 Theory. By A Correspondent. 



'T'HE undulatory theory of light has now been shown to 

 -■■ apply with such distinguished success to every phaeno- 

 menon, even the most recondite and complicated which the 

 researches of physical optics have disclosed, that no other 

 doubt can remain on the mind of the competent examiner of 

 its doctrines than that which results from the single exception 

 to its universal application ; viz. the explanation of the phce- 

 nomenon of the unequal refrangibility of the different rays of 

 light. 



In regard to this apparent exception several suggestions 

 have been made possessing high claims to attention. With- 

 out discussing their merit, it may be permitted to the writer 

 of these lines to propose another which seems to him more 

 free from objections than many of those which have pre- 

 ceded it. 



It will be admitted that the same difficulty which attaches 

 to the explanation of the modifications which the undulations 

 midergo within the refracting medium, applies to the concep- 

 tion of their condition in the medium out of which they enter 

 it. If we have only one homogeneous medium, it seems im- 

 possible to conceive more than one kind of undulation going 

 on at the same time in it ; the elasticity being an essentially 

 constant element. If we could by any possibility conceive dif- 

 ferent elasticities coexisting, and by consequence, vibrations 

 impressed with different velocities giving rise to undulations 

 of different lengths at the same time, these would of course be 

 unequally retarded on entering the denser medium, and un- 

 equal refraction would take place. 



The suggestion now offered consists in this, — that such a 



coexistence 



