124 On a periscopic Camera Olscura 



cipitate, the appearance of which differs from this in every 

 respect. 



By this method, I believe that every objection to the test 

 \\]\\ be removed ; and in order to anticipate all ani'cigiiity, 

 and to avoid any complication or practical difficulty in its 

 application, I would propose to njodify the process in the 

 followi'.uj; manner: 



To the suspected fluid, previously filtered, add, first, a 

 little dilute nitnc acid, and, afterwards, nitrat of silver, till 

 it shall cease to produce any precipitate. The muriatic 

 acid being thus removed, whilst the arsenious ac'.d (if any, 

 and in whatever state,) remains in the fluid, the addition of 

 ammonia will instantly produce the yellow precipitate in 

 its characteristic form, it is hardly necessary to add, that 

 the quantity of amnjonia must be sufficient to saturate any 

 excess of nitric acid which the solution may contain. 



XIX. On a PerUcop'ic Camera Ohscura and Microscope. 

 By William Hv^de Wojllaston, M.D. Sec. R.S.* 



Although the views which I originally had of the ad- 

 vania,f, to be derived from the periscopic construction of 

 spectacles t nrturpJly sucr^.^esied to me a corrcsponJing im- 

 provement m ihe camera ohscura, by substituting a me- 

 niscus for the double convex lens, I have huhcrto deferred 

 making it known to others, except as a subject of occasional 

 conversation. 



Since in vision with spectacles, as in common vision, 

 the pencil of rays received by the eye in each direction is 

 small, the superiority (jf that form of glass, which disposes 

 all parts of it most nearly at right angles with the visual 

 ray, admits of distinct demonstration : but with respect to 

 the camera obscura, where the portion of lens requisite for 

 suff.cient illumination is of considerable magnitude, al- 

 though it is evident that some improvement n)ay be made 

 ill the distinctness of oblique images on the same principles ; 

 yet as the focus of oblique rays is far from being a definite 

 point, the degree in which it may be improved is not a fit 

 subject of mathematical investigation. 



I have therefore had recourse to experiments, in order to 

 determine by what construction the field of distinct repre- 

 sentation may be most extended ; and I trust the result 

 will be acceptable to this Society. I shall take the same 



* From the Philosophical Tran»action» for 1812, part ii. 

 I Phil. Mag. vol. xiTi. Nicholson's JournJ,vii. 143. 



opportunity 



