On the Numkr of the Primitive Colorize Rays in Solar Light. 393 



towards the centre, and let thefe three divlfions be refpeftively painted red, yellow, and 

 blue, in proportions to be afcertained by trial; then if the annulus be turned fwiftly round 

 its centre, it will appear white. That white may be generated by the mixture of only the 

 three colours red, yellow, and blue, might alfo appear from the rule which Newton him- 

 felf has 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 diftinguiflied into feven arches propor- 

 tional to the feven mufical intervals in an oflave, that is, proportional to the numbers 45, 

 27, 48, 60, 60, 40, 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 co- 

 lour in the given mixture be defcribed. Find the commoh centre of gravity of all thefe 

 circles, and if this common centre of gravity coincide with the centre of the circle, 

 Newton fays that the compound 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 conftrudkion 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 of the red circle ; and if the number 

 of red rays be to the fum of the yellow and blue rays inverfely as the diftances of the centre 

 of the red circle, and the common centre of the yellow and blue from the centre of the 

 principal circle, the common centre of gravity of the red, blue and yellow circles will 

 coincide with the centre of the principal circle, and therefore the refulting compound will 

 be white. 



But it is manifeft that this conftrudtion cannot be relied on, becaufe the quantities of the 

 rays of any given colour in folar light, do not appear to be proportional to the fpaces 

 which they occupy in the rectilineal fides of the fpectrum. Thus it is known that the 

 yellow making rays are predominant in folar light, yet the fpacc they occupy in the fpec- 

 trum is to the fpace occupied either by green or blue ^ four fjp five, and to t\^ fpace qc- 

 cupied by the violet onlj: as three to fivf* 



VqIt. IV.— December 1800. 3 E Jn 



