s8i 
the Cause of coloured co?icentric Rings. 
The same experiment may be repeated with the red bow ; 
but as here the first surface is essential to the formation of the 
bow, the plain side of the convex lens or concave glass, when 
placed against the prism, as before, will produce streaks ; 
neither the bow, nor its streaks however will be in the least 
affected by the convexity or concavity of the outward surface 
of the glass applied, through which the light is admitted. A 
scattering glass will have no effect to disturb the bow or its 
streaks, and when this glass is emeried on both sides, we have 
again the bow complete, but without streaks ; and by this fact 
it is proved, that unless a polished plain reflecting surface is 
applied to the prism, streaks cannot be formed. 
47. The Streaks zvhich may be seen in the blue Bozv contain the 
Colours of both the Parts of the prismatic Spectrum, by the cri- 
tical separation of zvhich the Bozjo is formed. 
The moat favourable way of observing the colours of the 
blue bow streaks that are formed when a plain glass is laid 
under the base of a right angled prism, is to place a screen of 
white paper, before an open window, and to let the direct solar 
light shine through it upon the side of the prism. This scat- 
tered light will be bright and uniform, and cause no adventi- 
tious colours to mix with the streaks. The eye should be 
within six or seven inches of the prism. A streak consists of 
a certain principal colour and the intermediate tint which se- 
parates it from the next ; and in the following memorandum 
of fourteen streaks, which I saw in the manner above described, 
the principal colours are placed in front, and the dividing tints 
at the side between them. 
