493 



of plates requisite for complete polarization of the transmitted beam 

 is 30 : and since, under these circumstances, the whole of the light 

 that is not reflected at the first surface is transmitted through the 

 whole series, the author observes, that transmission is not in this 

 case a maximum at a perpendicular incidence, and that the law 

 employed by Bouguer fails by reason of these newly- discovered 

 properties of light, of which that distinguished philosopher was not 

 aware. 



The celebrated discovery of Malus, of the polarization of light by 

 oblique reflection, and its connexion with the properties of doubly- 

 refracting crystals, is perhaps the most important discovery that has 

 been made in optics since that of the principle of the achromatic 

 telescope ; but the author observes, that it does not furnish us with 

 any information of the manner in which these crystals effect polari- 

 zation, and that the present discovery of polarization by oblique re- 

 fraction supplies the connecting link between these two classes of 

 facts, and holds out a prospect of a direct explanation of the leading 

 phenomena of double refraction. 



Should the present paper meet with the approbation of the Society, 

 Dr. Brewster promises a further communication of experiments on 

 the polarization of light by reflection, in which he designs to show 

 that the law observed by Malus is not general, and that the principle 

 has been completely overlooked by him ; as it depends on the pro- 

 portion which the quantity of light reflected bears to that which is 

 transmitted when incident at the polarizing angle. When light is 

 incident upon water at the polarizing angle, he remarks that only 

 -r-iro-v is reflected ; that even from glass only r g is reflected ; but 

 when realgar, diamond, or chromate of lead are employed, then at 

 the polarizing angle these bodies reflect as much as one half of the 

 light, and consequently have not power to polarize all that they re- 

 flect. 



Further Experiments on the Light of the Cassegrainian Telescope com- 

 pared with that of the Gregorian. By Captain Henry Kater, Bri- 

 gade-Major. In a Letter addressed to the Right Hon. Sir Joseph 

 Banks, Bart. K.B. P.R.S. Read November 18, 1813. [Phil. 

 Trans. 1814,^. 231.] 



The experiments detailed in the present letter were conducted 

 exactly in the same manner as those detailed by Capt. Kater in his 

 former communication, for the purpose of comparing a new Casse- 

 grainian telescope, made by Mr. Crickmore of Ipswich, with the 

 Gregorian used in the former experiments. The diameter of the 

 large speculum in this instrument is 4'9 inches, but was reduced by 

 a ring of pasteboard to 3 -6, in order to render the illumination equal 

 to that of the Gregorian, in which the large speculum measured 

 3*95 inches. 



The areas exposed to the light being estimated at 7'152 and 

 10-593, and the magnifying powers at the same time being 157 and 



