red focus. This can be shewn byplacing a somewhat strong convex lens in the path of a beamfrom a brilliant source of light, and receiving the emergentlight on a screen. Thus, take the gas flame as the source oflight, hold the lens in such a position as to form an imageon the screen, and focus the… … 113-114] THE SPECTRUM. COLOUR. 191 with violet. The cause of this is seen from fig. 132. Theviolet rays converge to a pointF on the axis, the red to apoint R. A violet image isformed at F, and a red oneat R, If the screen be heldat F there is no violet lightin the outer part of theimage. It…
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