264 Proceedings of the British Association. 



upon a plan which seemed likely to conduct to the solution of this 

 problem — a problem which must be solved, before we can ascend with 

 certainty to the physical causes of the phenomena. 



'On a New Polarity of Light, with an Examination of Mr. Airy's 

 Explanation of it on the Undulatory Theory,' by Sir D. Brewster. — 

 Notwithstanding the great power of the Undulatory Theory in explain- 

 ing phenomena, and its occasional success in predicting them, I have 

 never been able to consider it as a representation of that interesting 

 assemblage of facts which constitute Physical Optics. When a theory 

 of high pretensions, and remarkable for its powers of accommoda- 

 tion is found incapable of explaining whole classes of well-observ- 

 ed and distinctly marked phenomena, those who have discovered 

 or studied these phenomena may be excused for withholding from 

 it their assent, and for not wholly abandoning older, though less 

 popular views, which were sanctioned by such authorities as those 

 of Newton and Laplace. It has fallen to my lot to lay before 

 the public several of the facts to which I refer ; but as it is not the 

 object of this notice to discuss the general merits of the Undulatory 

 Theory, I shall mention only two of those classes of facts which the 

 Undulatory Theory has failed to explain. The first of these, which 

 was communicated to the Royal Society about fifteen years ago, em- 

 braces the phenomena of transverse fringes which cross the fringes 

 produced by grooved surfaces, and produce, both in common and 

 homogeneous light, a series of phenomena equally beautiful and sin- 

 gular. In these phenomena we witness the extraordinary fact, that 

 a stripe of polished metal is incapable, at various angles of incidence, 

 of reflecting a single ray of homogeneous light ; while, at intermediate 

 angles of incidence, it reflects that light freely. The Undulatory 

 Theory has never ventured to explain these phenomena, and I feel 

 confident that they are beyond its power ; and hence the phenomena 

 themselves have excited no notice, and have shared the fate of all 

 such intractable notions as repose submissive to the prevailing theory 

 of the day. The second group of phenomena which the Undulatory 

 Theory is equally incapable of explaining, present themselves in 

 looking at a perfect solar spectrum, or a diffractive spectrum, through 

 the edge of a thin plate of glass, quartz , or mica. If we cover one- 

 half of the pupil of the eye with such a plate, and thus view the 



