£84 jD/yHerschei/s Experiment ts Fo ■ • investigating 
rays, of which that part is composed, are those which by re- 
flection re-enter the prism. 
48. On the Formation of Streaks . 
As I have now ascertained that the streaks we see when a 
plain glass is laid under a prism, which shows the blue bow, 
are formed by the principle of reflection, which throws back 
the transmitted rays, it will be a considerable satisfaction if 
we can trace the course of these rays far enough to have some 
idea of the arrangement, whereby such appearances may be 
^produced. To show, by calculation, the complete formation 
of the streaks in a case that is liable to such variation, on ac- 
count of the different contact between the modifying surfaces, 
the position of the light and the inclination of the eye, would 
be a most laborious, if not endless, undertaking ; it will there- 
fore be sufficient, if I can make it appear, that streaks must 
unavoidably be produced by the rays which after transmission 
are reflected back again, and mix with those that form the 
bow ; and this I believe will not be difficult. For instance, 
let F G, fig. 12, Plate XIII. be the base of a solid piece of glass, 
in which a compound ray of light is moving from A to «, with 
an obliquity A«G = 49 0 46' 12, "5 ; and let I K be the plain sur- 
face of a reflecting substance placed under the base ; then will 
the violet, indigo, blue, and the faintest part of the green of this 
ray be reflected at «, and the remaining green, the yellow, 
orange, and red will be transmitted. Now, in order to un- 
derstand the intention of this figure, it will be necessary to 
observe that on account of the minuteness of the operations of 
" light, all the lines and distances are represented upon a scale 
one thousand times larger than what the calculation gives 
