1820.] Proceedings of Philosophical Societies. 55 



which IS to be rendered most distinct. If it is required to make 

 the two images equally distinct, the refractive power of the 

 cement must be a mean between that of the ordinary refraction 

 and the extraordinary refraction, which corresponds to the angle 

 which the refracted ray forms with the axis of double refraction. 



5. All doubly refracting crystals consist of an ordinary and an 

 extraordinary medium alternating with each other, and vaiying 

 in density according to a law which the author has investigated, 

 but not given in this paper. Dr. Brewster, in the subsequent 

 part of this paper, shows that the change in the angle of polari- 

 zation produced by the interior force, depends on the inclina- 

 tion of the reflecting surface to the axis of the crystal, and also 

 on the azimuthal angle which the plane of reflection forms with 

 the principal section. 



This half volume terminates with a postscript to Dr. Young's- 

 paper, containing an investigation of the corrections for refrac- 

 tion. This investigation being entirely analytical, I must refer 

 those readers who wish to know the important results obtained 

 by this very acute philosopher to the paper itself, which occupies 

 only four pages. 



The usual meteorological journal for 1818 is given likewise in 

 this half volume ; the mean results of which are as follows r 

 temperature, 53*5° ; barometer, •29*88 inches, at the height of 81 

 feet above low water at Somerset House. No correction for 

 temperature is introduced, which renders the barometrical heights 

 given in the journal of little or no value. 



Rain, 11'636 inches. 



Mean variation of the needle in June, 24° 15' 43" W. 



Dip about 70° 51'. 



Article VIII. 

 Proceedings of Philosophical Societies. 



ROYAL SOCIETY. 



Nov. 25. — Dr. Carson's paper, On the Elasticity of the Lungs, 

 was concluded. After some introductory remarks, comprehend- 

 ing a popular description of the thorax and its contained viscera, 

 the author proceeded to observe that the influence of the elasti- 

 city of the lungs on the circulation of the blood and on respiration, 

 has been overlooked by physiologists. To ascertain the real 

 force of the elastic power of these organs, Dr. C. connected with 

 the tracheae of several animals a glass syphon, so placed as to 

 admit of pressure being exerted on the luugs by a column of 

 water contained in it. An opening was then made into the 

 cavity of the ckest on both sides, and the height of the column 

 of water in the tube was considered as equivalent to the pressure 



