is the morerefracted; thus the violet focus is further away than the red. Now let the light after passing through a convex lens fallas a convergent pencil on a concave one. The convex lenstends to bring the violet focus nearer to the lens than the red;the concave lens has the reverse effect. It may… … Fig. 133. bring the violet rays to V, the red to R. The concave lensBAG has an opposite effect, and the two foci coincide at F. 114-116] THE SPECTRUM. COLOUR. 193 There is one case in which this end can obviously beattained. If the two lenses be of the same material and havethe same focal length,…
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