34, 



if the second plane of incidence be at right angles to the former, then 

 no part of that ray will be reflected by the second surface ; and an 

 eye rightly placed for witnessing the effects, will perceive in the 

 exact specific direction a central spot of absolute blackness, sur- 

 rounded also by a dark space of some extent, from which less or more 

 of light is reflected, in proportion to the increase of distance from the 

 central line of no reflection. The light, however, which has thus 

 been polarized, may be wholly, or in part, depolarized by the inter- 

 position of many crystallized bodies, the degree of depolarization 

 being dependent on the more or less exact position of a certain neu- 

 tral axis or plane of their crystalline texture with the plane of pri- 

 mitive polarization. When these planes are perfectly coincident, the 

 light remains polarized in that plane, and a black line appears in 

 that direction ; but adjacent to it, on each side, are seen a series of 

 colours, which depend partly upon their proximity to the central 

 black line, and partly upon the thickness of the depolarizing body, 

 the succession of colours being exactly the same as those observed 

 by Newton in thin plates, but variously modified in their forms, ac- 

 cording to the nature of the crystalline substance interposed, and ac- 

 cording to the position of its axis. 



The optical effects of heated glass, as now observed by Dr. Brewster, 

 are precisely of the same kind, and are now found to depend not 

 upon the simple circumstance of temperature of the entire plate, as 

 he originally supposed, but upon the progressive differences of tem- 

 perature in different parts of the plate, arising from contact or proxi- 

 mity to a plate of hot or cold iron, or from the cooling power of the 

 surrounding atmosphere. And in the same manner as the several 

 tints of colour produced by crystallized bodies, have been shown by 

 M. Biot to depend on a series of thicknesses proportional to those in 

 Newton's scale for thin plates ; so with respect to heated glass, Dr. 

 Brewster observes, that a corresponding arithmetical progression is 

 observable for the same tints, whether the thicknesses compared be 

 those of single plates, or the aggregate thickness of several com- 

 bined. 



From these phenomena, Dr. Brewster infers the production of 

 what he calls a crystalline structure in the glass during its contact 

 with heated iron ; but observing the existence of an opposite struc- 

 ture in the middle of the glass, and again the same structure at its 

 remote extremity apparently beyond the reach of sensible heat, he 

 says there is nothing analogous but in the perplexing phenomena of 

 magnetic and electric polarity. 



In the prosecution of these experiments, the author varies indefi- 

 nitely the forms and dimensions of his plates of glass, and with them 

 the forms of the fringes produced ; but it would be next to impos- 

 sible to convey any correct idea of the various appearances without 

 assistance from drawings ; neither indeed could Dr. Brewster himself 

 have observed the phenomena with sufficient precision in their fluc- 

 tuating state as arising from the temporary communication of heat, 

 had he not found means to render the same properties permanent, 



