on the screen. The same end may be attained by forming the spectrumas in fig. 127, and then allowing the rays, after traversing the 188 LIGHT. [CH. IX prism to fall on a convex lens LMN, fig. 130. The rays ofany one colour are now diverging from one point of a virtualimage of the slit along v, y,… … are found by joining v and r to the centre M of the lens, asshewn by the dotted lines in the figure, YMy is the axis ofthe lens. To determine the position of this spectrum, it is bestto replace the lamp by the sodium flame, and to adjust thescreen, keeping it approximately perpendicular to the…
Heat · Light
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