130 RnyaL Sociehj. 



eye hole which is on the left. By this method of diminish- 

 ino; with the Diacatoptron, it is easy to take in prospects 

 and objects in perspective. 



Make a board (fig. 8.) 2,^ feet long, and bore two^ 

 holes six inches from one end of the board at the width of 

 the Diacatoptron, so that the supports of the table may 

 pass through and may be placed upright. Fasten behind 

 two perpendicular pieces of wood a ; and also at the upper 

 part two props which may pass into the holes of the table, 

 into which the feet were before fixed, so that the table may 

 be .ecured both at the top and boltoni, and all shaking 

 prevented. 



Bore holes in the board from c to a, that you may place 

 tliere the siaht-piece e, which is either made of brass or 

 wood, aboul six inches high, and has at the top a round 

 brass plate one inch in diameter, in which is the eye hole. 



When you make a trial of it, ^iv. the paper last on the 

 table according to lig. 9. Place the Diacatoptron about 

 six or eight inches from it, upon the board, and parallel 

 with the table; the blind, (fig. 10.) which is made of 

 wood, pasteboard, or j)aper, then covers the space between 

 the Diacatoptron and the table, so that the opening g may 

 be on the ri^ht hand : the sight-piece e is then so placed 

 in one of the holes that you may have the desired power ot 

 diminishing, taking care that your head may not intervene 

 between the object and the glass, and you may easily draw 

 upon the perpendicular paper the objects diminished. In 

 this way yh\i may with ease draw plants, and in short all 

 objects, even academy figures, &:c., and under circum- 

 stances in which the Camera obscura could not be con- 

 veniently employed. 



XXI. Proceedings of Learned Societies. 



ROYAL SOCIEXr. 



Thursday Feb. 6. JL he Society again met, when a short 

 paper by Dr. WoUaston was read on the primitive form of 

 calcareous, bitter, and iron spar. The composition of these 

 three kinds of spai- being essentially difl~ercnt, although their 

 primitive form has hitherto been admitted to be the same, 

 seemed to militate against the truth of the crystallogica! 

 system. Haiiy himself having rather hastily allowed that 

 the angle of the ihomboid of calcareous spar, magnesian 

 spar, and iron spar, being 101° 27', formed an exception to 

 the general principle that a difference of constituent parts 

 produced a difference of form, Dr.W. with his goniometer 

 was induced to examine these spars with greater attention. 



The 



