i68 Dr. Herschei/s Experiments for Investigating 
oblique incidence of 49° 49' 21"; and since we know that 
many of the rays which fall on the same spot, will be trans- 
mitted while others are reflected, we find by computation, 
that blue rays with the above incidence being transmitted, 
will be refracted in such a manner, as to arrive at the sub- 
jacent reflecting surface in an angle of 1T32", and will, 
therefore, re-enter the prism at the magnified distance of 
59,6 inches from the place at which they left it, or 63,2 from 
«; and this will give an extent to streaks, amounting to nearly 
2 ^ times the breadth of the bow. The same will take place 
with indigo and violet rays to an indefinite extent, which it 
will not be necessary to particularise. 
Third cause. In the foregoing article it has been shown, 
that beside the single reflection, which in the 14th figure is 
delineated as taking place between the base of the prism, and 
the subjacent reflecting plane, 6 , 7, and 8 succeeding reitera- 
tions of the same effect will carry on the reflected rays to a 
certain extent which is assignable. These rays would have 
crowded the figure so much, that they could not be inserted ; but 
let us see how far they may extend the breadth of the streaks. 
I have already shown, that the first reflection, on the magni- 
fied dimensions of the figure, will carry the transmitted rays 
2,64 inches beyond the bow ; these rays, by reiterated reflec- 
tions, may therefore be extended to 14,84; 18,48, or 21,12: 
inches beyond it, which alone will be sufficient ; but if more- 
over, the intermediate rays are here also taken into consi- 
deration, there cannot remain a doubt, but that the breadth 
of the streaks is sufficiently accounted for. 
