Onanative ArseniateofLead, Bf 



pil becomes larger, the circle of dissipation increases, and 

 consequently remote objects appear less distinct than they = 

 did in tlie early part of life. '. ri 



VI.— From this investigation it manifestly appears, tl^at 

 the eye is in reality no more than a michine of a fixed and., 

 determinate form, without any power to alter iis outward^ 

 dimensions, or to move any of its internal humours ; and'- 

 that the only adjustment necessary to form a distinct pic- ! 

 ture of an object upon the retina (whether the object be ^ 

 near or remote) is to prevent diverging ray^ from entering' 

 the eye, and to admit such onlv as are nearly parallel. , ' 



This office is performed by the iris, which contracts th« 

 pupil to exclude the side rays when we view near objects, 

 and enlarges this aperture to give us a distinct view Qf such 

 remote objects as are but faintly illuminated: it is also 

 well known, that whenever the eye is exposed to a strong 

 light, the pupil contracts, but it expands as the light de- 

 creases. 



These contractions and dilatations of the pupil, according ' 

 to the distances of objects and strength of light in which 

 they are seen, are directed bv that volition of the mind 

 which presides over and regulates all the other motions of 

 the eye. 



Ez, Walker, 



Lynn, Januarys, 1810. • 



XIV. On a native Arsenmte of Lead, By the Reverend 

 William Gkegor Comrmmicated by Charles Hat-" 

 CHET, Esq,, F.jR.S.* 



I. 



JL hat the oxide of lead and the arsenic acid might be 

 found in the state of natural combination, is a supposirioa 

 highly probable, from the strong affinity which subsists, 

 between these two substances. But the existence of such a 

 compound has not, as I conceive, hitherto been established.' 

 by such proofs, as entitle it to be ranked amongst the der-, 

 cided cases of mineralogical science. I trust, thereforCj,, 

 that the observations, which I have the honour of sjibmit- , 

 ting to the Socie y, on a new f ore of lead lately discovered 



* From Philosophical Transactions for 1809, Part II. 



f It isnevv at least to the miners in Cornwall; nor was there, previously 

 to this discovery, any are resembling it to be found in that splendid collec- 

 tion of minerals, which my valuable friend Philip Rashleigh, esq., has fo 

 liberally formed, and as liberally employed in th« promotion of science. 



I' 4 i;^ 



