General View of an Oscillatory Theory of Light. 403 



is liable to become spoiled, it is safest in observations on the 

 reflected light to make use of a crystal recently taken out of the 

 mother-liquor. I have only observed four bright bands in the 

 reflected light, whereas there are five distinct minima in the 

 light transmitted by the solution. However, the extreme minima 

 are less conspicuous than the intervening ones, besides which 

 the fifth occurs in a comparatively feeble region of the spectrum. 

 The fourth bright band in the reflected light was rather feeble, 

 but with finer crystals perhaps even a fifth might have been 

 visible. As the metallicity of the crystals is almost or perhaps 

 quite insensible in the parts of the spectrum corresponding to 

 the maxima of transparency, we may say, that, as regards the 

 optical properties of the reflected light, the medium changes 

 four or five times from a transparent substance to a metal and 

 back again, as the refrangibility of the light changes from a little 

 beyond the fixed line D to a little beyond F. 



Pembroke College, Cambridge, 

 November 14, 1853. 



LXII. General View of an Oscillatory Theory of Light. By 

 W. J.Macquorn Rankine, C.E., F.R.SS. Lond. andEdinb.fyc* 



1 . Difficulties of the present Hypothesis. 



NOTWITHSTANDING the perfection to which the geome- 

 trical part of the undulatory theory of light has been 

 brought, it is admitted that great difficulty exists in framing, to 

 serve as a basis for the theory, a physical hypothesis which shall 

 at once be consistent with itself and with the known properties 

 of matter. 



The present paper is a summary of the results of an attempt 

 to diminish that difficulty. All the conclusions stated have been 

 deduced by means of strict mathematical analysis ; and although 

 it is impossible to read the investigations before the British 

 Association in detail, their results can easily be verified by every 

 mathematician who is familiar with the undulatory theory in its 

 present form. 



It may be considered as established, that if we assume the 

 supposition that plane-polarized light (out of the varieties of 

 which all other light can be compounded) consists in the wave- 

 like transmission of a state of motion, the nature and magnitude 

 of which are functions of the direction and length of a line trans- 

 verse to the direction of propagation, we can deduce from this 

 supposition, with the aid of experimental data, and of certain 

 auxiliary hypotheses, the laws of the phenomena of the inter- 



* Communicated by the Author, having been read to the British Associa- 

 tion for the Advancement of Science, Section A, at Hull, September 10, 1853. 



