bTCOMPGSITlOJf OF LIGHT; 25 



The following is the manlier in which we proceeded, and 

 its results. 



It is hot easy to procure a single lens, that shall be ca- Difficulty of 

 pable of giving a focus of ten or eleven feet in the position £ eUin g a 8°°° 

 in which Newton employed it; for several glasses of little 

 curvature, that were lent me as fit for the purpose, were 

 altogether incapable of effecting it. I then imagined, that ^ 



I might succeed by placing near the shutter an object glass Resource of the 

 of short focus, to make the pencil very divergent, beyond aut 



it; and placing at the same time at a sufficient distance, 

 an excellent lens of Mr. Tremery's of five feet focus. 



The effect answered our wishes, and in consequence we His apparatus 

 arranged our apparatus as follows: 1, on the outside of 

 the windows, a plane metallic speculum, to reflect the solar 

 image: 2, an object glass of 87 centimetres (33 inches) 

 focus, distant from the speculum about 24 centimetres 

 (9, 36 inches): 3, a diaphragm, pierced with a hole six 

 millimetres in diameter (2.36 lines,) and at the distance of 



II centimetres (4." 3 inches) from the object glass, to in* 

 troduce the pencil of light into the room : 4, a lefts of 162 

 centimetres (5 feet y 3 inches focus, placed 32 centimetres 

 (1 foot) from the object glass: 5, at 11 centimetres (4. 3 

 inches) from the lens a prism of very clear flint glass, with 



angles of 60°, covered with black paper on each side, ex. ' 

 cept at the place left for the transmission of the rays; this 

 prism being continued so as to be moveable in different 

 directions, as occasion might require: 6, a board covered 

 with white cloth, at the distance of 422 centimetres (13 

 feet, 8 inches) from the lens. All these were placed, kept, 

 or brought into the proper directions, suited to their several 

 purposes, and to the course of the sun. The place too wa3 

 so contrived, as to be rendered pretty dark at pleasure. 

 Having taken every possible care in arranging our appa- 

 ratus, we were able to obtain every day, Mhen it was fine <■■ ' 



weather, a very simple spectrum for several hours; 

 which was quite sufficient for our various experiments, at 

 some of which Messrs. Berthollet, the father and son, Mr. 

 Laplace, and other gentlemen were present. 



The spectrum was very distinctly bounded by two rec- ^ ver d -, H 

 tiiinear, and perfectly parallel sides. Jts lengtfe was a iittle spJcuum 9J* 



inches long 



