On the Tkcomposition of Light. 213 



by their concurrence, when the rays arrive at first in parallel 

 directions ; still more it' they arrive diverging, as they do 

 when a pencil of light is admitted through a simple hole in 

 the window-shutter of a dark room ; in which case the sen- 

 sible diameter of the sun's disk must occasion a divergence 

 of the pencil. There is only one circumstance favourable 

 therefore, that in which the rays may be rendered conver- 

 gent, without-infringing the preceding conditions. 



The only method of doing this did not escape the sagacity 

 of Newton. He effected it by placing at a considerable 

 distance from the shutter, and but a little before the prism, 

 a lens of a long focus, which by its position regulated the 

 distance of the plane on which the spectrum was to be re- 

 ceived. In this way, and by the assistance of some other 

 precautions, he resolved this grand problem in optics. 



The following is the manner in which we proceeded, andf 

 its results. 



It is not easy to procure a single lens, that shall be ca~ 

 pable of giving a focus of ten or eleven feet in the position 

 in which Newton employed it ; for several glasses of littlfi 

 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 

 of short focus, to make the pencil very divergent beyond 

 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 wc 

 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, Sfi inches) : 3, a diai)hragm, pierced with a hole six 

 iTiillimetres in diameter (2'36 lines,) and at the distance of 

 11 centimetres (4* 3 inches) from the object glass, to in- 

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

 centimetres (.5 feet, 3 inches focus), placed 32 centimetres 

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

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

 angles of Gu'^, covered with black paper ou each side, ex- 

 O 3 ecpt 



