6 Sketch, of the Improvements in Science [Jan. 



death be ascertained that polished metals polarize light as well as 

 other substances : a discovery which was also made by Dr. Brew^ 

 ster, without his being aware that he had been anticipated by the 

 French philosopher. 



When a ray of light was divided into two pencils by a rhomboid 

 of Iceland spar, Malus made these pencils fall on a surface of water 

 at an angle of 52° 45'. When the principal section of the rhom- 

 boid, (or the plane which bisects the obtuse angles) was parallel to 

 the plane of reflection^ the ordinary pencil was partly reflected 

 and partly refracted, like any other light; but the extraordinary 

 ray penetrated the water entire, and not one of its particles escaped 

 refraction. On the contrary, when the principal section of the 

 crystal was perpendicular to the plane of reflection, the extraordi- 

 nary ray was partly refracted and reflected, while the ordinary ray 

 was refracted entire. 



M. Arrago observed the singular alternations of colour exhibited 

 by plates of mica, selenite,^and rock crystal, when they are exposed 

 to a polarized ray : and M. Biot has discovered the exact laws of 

 these phenomena, has expressed them by mathematical formulas, 

 and has reduced them all to one general fact, from which all the 

 phenomena may be reduced by calculation. For an account of the 

 labours of Biot in this department of optics we refer to the Annals 

 of Philosophy, vol. i. p. 225, 



Dr. Brewster's researches have been published in his Treatise on 

 New Philosophical Instruments,* the fourth book of which appears 

 to us by far the most ingenious and important part. Several of the 

 tables of experiments there given are" well entitled to the attention 

 of philosophers. We must satisfy ourselves at present with laying 

 before our readers Dr. Brewster's own account of the results of his 

 researches. 



" 1. It has been ascertained that chromate of lead and realgar 

 have a greater refractive power than the diamond, whicli has always 

 been supposed to exceed every other body in its action upon light. 



" 2. The chromate of lead possesses a double refraction, about 

 thrice as great as that of Iceland spar. 



" 3. The three simple inflammable substances have their refrac- 

 tive powers in the very order of their inflammability. 



" 4. All doubly refracting crystals possess a double dispersive 

 power, the greatest refraction being accompanied with the highest 

 power of dispersion. 



" 5. The fluates, viz. fluor spar and cryolite, have the lowest 

 refractive powers of all solid substances, and the lowest dispersive 

 powers of all bodies. 



" G. The agate, when cut by a plane at right angles to the laminae 

 of which it is composed, impresses upon a transmitted ray of light 



• Wecxpecfrd to have bern able before this time to notice tliii work in the 

 Jinnals of I'hilosophi/ ; but a pressure of matter has hitherto put it out of our 

 -power to present our readers with an analj'tis of it. 



