VOL. XXIX.J PHILOSOPHICAL TRANSACTIONS. 329 



sufficient directions in Sir Isaac Newton's Optics; though I had no other 

 directions than what I found there. 



A plain and easy Experiment to confirm Sir Isaac Newton's Doctrine of the 



different Refrangibility of the Rays of Light. By the same. N° 348, 



p. 448. 



After the experimentum crucis made by two prisms, I should not give the 

 following experiment, but that it is so easy to be made, that by it those who 

 want the apparatus, or are unwilling to be at the pains to make the experi- 

 mentum crucis, may at any time satisfy themselves of the truth of the fore- 

 mentioned doctrine. 



Let the candle a be set before the bar of a chimney looking-glass, such as is 

 represented by hh, fig. 25, pi. 6, which is a piece of looking-glass plate con- 

 sisting of four planes, seen in the section of it afdj3, viz. d|3 which is quick- 

 silver behind, f<x a plane parallel to it, fd one of the side-planes bevelled to- 

 wards dj3, or inclined to it in an angle of about 40°, (though from 30 to 40 

 will do, but the greater the angle the better, if it does not exceed 45°), a|3 the 

 other side-plane inclined in the same angle to |3d. 



The rays of the candle which come from a to y, fall obliquely on the plane 

 a(3, so that instead of going on to a, they are by refraction made to incline 

 more towards the perpendicular pp, namely to go on in the line yc, and then 

 are reflected from the point c on the quicksilvered surface, in the direction cx, 

 so as to make the angle xcd = ^cp. Now as the rays which would go to x, if 

 not refracted, emerge obliquely from the plane a(3, they leave the direction cx, 

 and decline from the perpendicular tttt, and, being difierently refracted, open 

 into four differently coloured rays ; viz. bn a red ray, byo a ray made up of 

 orange and yellow; bGB a ray made up of green and blue or a sea-green, and 

 bp a purple ray. 



If from the place ec you look full on the point b, the spectrum or image of 

 the candle at b will appear double, but not mixed ; that is, there will appear a 

 sea-green spot and a red spot, as it were one upon another; but not so as to 

 produce a mixed or intermediate colour. Then if the right eye or eye at e be 

 shut, there will appear only a green spot to the eye at e; if the eye at e be shut, 

 the eye at e will see only a red spot. 



If you come nearer to b, so that the eyes at £l, i1 receive the most and the 

 least refrangible rays, there will be a double spectrum, viz. a red and a purple 

 one just touching, or upon one another : and the phenomenon will answer as 

 before. (Fig. 25). 



