Mr, Ivory 07i the Theory of the Astronomical Refractions. 385 



sensitive character of his preparation is lost, and the value of 

 less sensitive modes is reduced in a still greater proportion. 



Daguerre informs us, that under very favourable circum- 

 stances a drawing may be obtained by exposing his plates in 

 the camera during from three to five minutes. If then, by 

 correcting his focus, he were enabled to use a lens of equal 

 power to the one by which the inclosed drawings were pro- 

 duced, he would be enabled to make the necessary impression 

 in from ten to twelve seconds. 



During the discussion which took place at the Institute, 

 after M. Arago had publicly announced the process of Da- 

 guerreotype, it was allowed to be a great desideratum that the 

 art might be applied to taking portraits from life. The use 

 of large lenses, which the correction of the focus enables us to 

 adopt, would, I should imagine, render such an application 

 of the art practicable; and the value of each use to which this 

 important invention is applied, must also be increased by a 

 knowledge of the means of obtaining the best possible effect 

 in the least possible time. 



I am, Gentlemen, your obedient servant, 

 John T. Towson. 



LIX. The Bakerian Lecture. — On the Theory of the Astro- 

 nomical Refractions. By James Ivory, XIH., M.A., F.R.S. 

 L. Sf E.y Instit. Reg. Sc. Paris, Corresp. et Reg. Sc. Gotiin, 

 Corresp, 



[Continued from p. 109.] 



Atmosphere of Air mixed with aqueous Vapour. 



/"CONTINUING to represent the pressure and temperature 

 ^-^ at the earth's surface by p and t', and the like quantities 

 at the height ^ by p and r, the symbols {/), (p) may be used 

 to denote the respective densities in the case of air mixed with 

 aqueous vapour. When the pressure and density vary, all 

 the gases, and mixtures of gases and vapours, are found to 

 follow the same laws of dilatation and compression ; and hence 

 the same equations that express the equilibrium of an atmo- 

 sphere of dry air, will hold equally in one of moist air. In 

 the present case these equations will therefore be, 



(-ly 



p. - 1+^^ (p1! . 



/ 1+/3t'*(p')- 



• This equation is equivalent to the one in p. 18 of M. Biot's disserta- 

 tion, on whicii that author lays so much stress. 



Phil. Mag. S. 3. Vol. 15. No. 97. Nov, 1839. 2 G 



