30 



Count Dournon on certain Minerals 



fore shall merely give a figure of what I conceive to be the 

 true theory of this instrument 



a, the letter T pasted on the window, sends an image to the 



convex object-glass, which is too much rarefied to pass to the 

 eve, but two inverted images are formed at b and c, which 

 uniting at the eye-glass CD, form one inverted imaged. This 

 again is magnified and sent to the eye at E, whence it travels 

 to the retina and sensorium. In both telescopes the image 

 at the eye-glass stands in place of an object. 



As this paper is intended as a mere outline, I have not 

 given the exact foci of the glasses, choosing those of high 

 powers. The experimenter is to distinguish between the con- 

 verging rays and those which form the image. Let him 

 make the following experiment : — Place the glass globe filled 

 with water in the sunbeams, and holding a convex lens over 

 it, he perceives an image of the lens with the sun painted on 

 the convex surface, and he also perceives the gross refracted 

 rays form an image of the sun which can be thrown to some 

 distance, having nothing whatsoever to do with vision. This 

 I proved by the following experiment : I desired a friend to 

 look at a lighted candle, and holding a convex lens opposite 

 his pupi 1 , I perceived the lens with a magnified image of the 

 candle to be painted on his pupil, while by holding the lens 

 a little obliquely, I threw the refracted rays on the side of his 

 nose or on his forehead. 



I remain your obedient servant, 

 Cork, April 22, 1823. Joseph Reade, M.D. 



VI. Description of certain Gangucs of Spindle brought from 



the Island of Ceylon by M. Leschenault oe Lato'uR. 



By M. le Comte de Bourn on, C/iev. de St. Louis, F. U.S. 



Sfc. * 



IN ii memoir on Corundum which I presented to the Royal 



■ Society of London, and which has been printed in the 



Philosophical Transactions for 1802, after having fulfilled the 



* Extracted from Observations sur quelques-uns des Mineraux, soil dc 

 V He de Ceylon, soit de la Cote de Coromandcl, rapporiet par M. Llschl,- 



mavlt dk Laiolji. Par M Le Comte de Bournon. Pans. 1823. 



end 



