1835.] Distinct Vision at Different Distances. 369 



size of the pupillary aperture, we might discover whether 

 any other action, besides that of the pupil, was necessary to 

 effect the change in question. With this instrument the 

 following observations were noticed. The pupil being of a 

 moderate size, a small object, placed within a few inches of 

 the cornea, was represented on the retina in a very shadowy 

 and indistinct manner. On contracting the aperture, which 

 was done by covering the iris and pupil with a piece of black 

 riband, perforated in the centre, the picture of the object 

 appeared beautifully clear and well defined. Now, here 

 there could be no vital action, no change in the figure or 

 position of the lens, nothing but the alteration in the 

 dimensions of the pupil, and yet the desired effect was 

 produced in the most satisfactory manner. With the same 

 instrument, it is demonstrable, that if the lens be removed, 

 and the pupil remain of the natural size, no object whatever 

 will be represented on the retina ; but, with a contracted 

 pupil, and the lens still absent, a very fair outline of any 

 object will be clearly and distinctly seen. 



Experiments of all sorts, I am well aware, generally ap- 

 pear more determinate and conclusive to their authors than 

 to indifferent individuals ; and, when they are anxious to 

 establish any particular theory, they do not easily see 

 objections, which to others are sufficiently apparent. It is 

 therefore not unlikely that I may have overrated the impor- 

 tance of these experiments, when I say that, to me, they 

 appear to settle the point at issue. If, however, I shall 

 have arrived at an unsound conclusion, I must then throw 

 myself back on the novelty and interest of the facts as my 

 only apology. 



Article VI. 



Account of some new Species of Minerals containing Barytes. 

 By Thomas Thomson, M.D., F.R.S., L. and E., &c, 

 Regius Professor "of Chemistry in the University of 

 Glasgow. 



The species of barytes minerals hitherto described by 

 mineralogists are but few in number. The sulphate and 

 carbonate have been long known. When to those we add 

 vol. i. 2 b 



