239 



their positions by an alloy of zinc and copper, as easily wrought as 

 common brass, and that they can be executed in this manner of any 

 required size ; that castings of the finest speculum metal can be ex- 

 ecuted of large dimensions, perfect, and not very liable to break ; 

 that machinery can be employed with the greatest advantage in 

 grinding and polishing specula ; that to obtain the finest polish, it is 

 not necessary that the speculum should become warm, and that any 

 temperature may be fixed upon, and preserved uniform during the 

 whole process ; and that large specula can be pohshed as accurately 

 as small ones, and be supported so as to be secured from flexure. 



2. On the theoretical explanation of an apparently new Polarity 

 in Light. By G. B. Airy, Esq., M.A., F.R.S., Astronomer Royal. 



The existence of a polarity in the rays of homogeneous light, ha- 

 ving regard only to the sequence of colours in the spectrum, was in- 

 ferred by Sir David Brewster from some experiments, of which he has 

 given an account, contained in the Report of the seventh meeting of 

 the British Association. The author states the results of his own 

 observations of similar phenomena, and their theoretical explanation 

 on the undulatory theory, together with the mathematical develop- 

 ment of that explanation. 



3. On the Ferrosesquicyanuret of Potassium. By Alfred Smee, 

 Esq. Communicated by P. M. Roget, M.D., Sec. R.S. 



The author examines, in this paper, the action of chlorine upon 

 the ferrocyanate of potassa, and the conversion of the latter into 

 ferrosesquicyanuret ; and proposes methods for obtaining this latter 

 salt uncontaminated with impurities, and free from the difficulties 

 and inconvenience attendant on the present mode of preparation. 



4. On the influence of Iodine in rendering several argentine com- 

 pounds, spread on paper, sensitive to light; and on a new Method 

 of producing, with greater distinctness, the Photogenic Image. By 

 Mr. Robert Hunt. Communicated by Sir John Herschel, Bart., 

 V.P.R.S. 



This paper contains various details of the results of a great num- 

 ber of experiments made with a view of rendering paper capable of 

 being employed instead of metallic plates, in Daguerre's photogra- 

 phic process. It is accompanied with 12 papers as specimens. 



5. Hourly Observations of the Barometer and Thermometer at 

 sea, on the 21st of March, 1840. By Major-General A. Lindsay, 

 H.E.I. C.S. Communicated by Sir John F. W. Herschel, Bart., 

 V.P.R.S. 



These observations were made on board the ship Owen Glendower, 

 on her voyage from Calcutta to London. 



6. On the Constitution of Pigotite, and on the Mudesous and 

 Mudesic Acids. By James F. W. Johnston, Esq., M.A., F.R.S. 



In this paper the author describes a substance, found by himself 



