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colours red, yellow and blue might alfo appear from the rule 

 which Newton himfelf uas given us, for determining the colour 

 of the compound which refults from the mixture of any primary 

 colours, the quantity and quality of each being given. 



The rule is this, the circumference of a circle is diilingulfhed 

 into feven arches proportional to the feven mufical intervals in an 

 o6lave, that is, proportional to the numbers 45, 27, 48, 60, 60, 

 4c, 80 : the firft part is to reprefent a red colour, the fecond orange, 

 the third yellow, the fourth green, the fifth blue, the fixth indigo, 

 and the feventh violet. Thefe are to be confidered to be all the 

 colours of uncompounded light gradually paffing into one another, 

 as they do when made by prifms, the circumference reprefenting 

 the whole feries of colours from one end of the fun's coloured 

 image to the other. Round the centers of gravity of thefe arches 

 let circles proportional to the number of rays of each colour in 

 the given mixture be defcribed. Find the common centre of gra- 

 vity of all thefe circles, and if this common centre of gravity coin- 

 cide with the centre of the circle, Newton fays that the com- 

 pound will be white. Join therefore the centers of gravity of the 

 blue and yellow circles, and from the centre of the red circle draw 

 a right line through the centre of the principal circle ; from the 

 conflru£lion it will cut the line which joins the centers of the blue 

 and yellow circles-, if therefore the number of the blue and yellow 

 rays be to each other inverfely as their diftances from the point where 

 the line which joins their centers is cut by the line drawn from the 



centre 



