[ 12.3 ] 



evident by obfervatlon of the fpecflrum, fince in it we perceive, that 

 the prifmatic colours are difFufed over fpaces, which are, on the 

 fides, terminated by right lines, and therefore the centers of the 

 circles of the fame denomination of colour are difFufed over lines 

 equal to thefe fegments of the reflilineal fides of the fpedlrum. 

 Newton has fliewn, prop. 4. B. i. Optics, how to feparate from one 

 another theheterogeneous rays of compound light, by diminifliing 

 the breadth of the fpedrum, its length remaining unchanged ; and 

 when the length of the fpedlrum is to its breadth, as 72 to i, the 

 light of the image is feventyone times lefs compound than the 

 fun's dired light. In the middle of a black paper he made a 

 round hole, about a fifth or a fixth part of an inch in diameter, 

 upon which he caufed this fpedrum fo to fall, that fome part of 

 the light might pafs through the hole of the paper ; this tranf- 

 mitted part of the light he refradled with a prifm placed behind 

 the paper, and letting the light fall perpendicularly upon a white 

 paper, he found that the fpedtrum formed by it was perfeclly eir- 

 eular. Hence, therefore, it follows, that the equally refrangible 

 rays occupy a fpace on the redilineal fides of the fpectrum equal 

 at leaft to the fifth or fixth part of an inch ; that is, the rays 

 of the fame colour are differently refrangible. 



The different quantity of the homogeneous rays of different co- 

 lours will not account for the different fpaces they occupy in the 

 fpedrum ; for this difference in quantity would aftedl only the 

 intenfity of the colour, not the magnitude of the fpace which it 



Q_ 2 would 



