e 66 Dr. Herschel’s Experiments for investigating 
4 1 . Particulars relating to the Newtonian prismatic blue Bow. 
The Newtonian blue bow may very conveniently be exa- 
mined, when a right angled prism is laid down on a table 
before an open window. The eye being then brought to a 
convenient altitude, and pretty near the side of the prism, we 
see in it a bow, which from the predominant colour may be 
called blue. It contains some green followed by blue, indigo, 
and violet. A very faint red, orange, and yellow may also be 
perceived above the greenish colour ; but these belong not to 
the blue bow, and have not been noticed by the author. Their 
appearance will hereafter be accounted for. 
To analyse this blue bow more particularly, let us admit 
that the colours, which give it the general appearance of what 
may be called blue, consists of half the green, and of all the 
blue, indigo, and violet rays, which are reflected while the 
other half of the green, the yellow, orange, and red are trans- 
mitted. Then the angle of obliquity, at which this separation 
of the colours will happen, in consequence of the different 
refrangibility of the differently coloured rays assigned by 
Newton, will be 49 0 4 6 ' 12", 5. 
Let ABCDE, Plate XII. Fig. 1, be rays of light mov- 
ing within glass in such directions, as to fall on the interior 
base F G upon the points a 1 Then, if it be required that 
these rays after reflection from the base should meet in the 
point H,and form the blue bow, the angles A «G, B/ 3 G, C^G, 
D $G, and E £ G must be respectively equal to 49 0 46' 12", 5 ; 
49° 49' 2 °" ; 49° 55' 2 > c f '$ 5 49° 59' 4 i ">4; and 5°° / 54" 5 
which will give the angles « H / 3 , 12 H y, y H £, $ H s, equal to 
3' 7" ,5, 6' 13" ,6, 4/ 7" 3 8, and 8' i2",b, making in the whole 
