Besaiptlon of the Camera Lucida. 3 43 



well as alkali, when the disks being dried the pile ceases to 

 disengage the dectrical fluid, the aliiiv.ty of the acid of the 

 positive pole with the alkali of the negative pole enters into 

 action, a muriatic salt is formed. 



The radier.l of the muriatic acid is therefore a fact as cer- 

 tain as it -s novel in chemistry, and for which we are in- 

 debted to M. Pacchiani ; as well as lor many others \\ Inch 

 have presented themselves almost si^ntaneously to otlier^ 

 but which no person except M. Pacchiani knew liow to lay 

 hold or. Kcir and Cavendish seem to have been in this 

 predicament, since they niennon a liule muriatic acid in the 

 water produced bv tlie combustion of hydrogen v. ith oxy- 

 ocn. withoul havmcr ascertained the trn- oiium of it. On 

 thi^ subject we may consult the E-i-ay on Static Chennstry, 

 by M. Btrlholletj part ii. sect. 1. chap. ii. 



L^'II^. Descripf inn of /he Camera Lncida. % William If. 



VV'OLLASTON, StC. R. S.* 



Having, a short time since, amused myself with attempts 

 to sketch various interesting views without an adequate 

 knowledge of tiie art of drawing, my mind was naturally 

 cmploved in lacilitating the means of transferring to paper 

 the apparent relative positions of the objects before me; and 

 1 am in hopes that the instrument which I contrived for 

 this purpose may be acceptable even to those who have at- 

 tained to greater proficiency in the art, on account of the 

 nianv advantages it possesses over the connnon camera 

 ohsci/ra. 



The principles on which it is constructed will probably 

 be most distinctly explained by tracing the successive steps 

 by which f proceeded in its formation. 



While I look directly down at a sheet of paper on my 

 table, if I hold between my eye and the paper a piece of 

 |)iain glass inclined from me downwards as an angle of 45'^, 

 1 see bv reflection the view that is before me in tiiC same 



* ComniuuicatcJ by the Aiiihor. 



dlrectioa 



3 



