Page:Elementary Text-book of Physics (Anthony, 1897).djvu/456

442 the secondary axis $$oR$$ of the eye-lens $$C$$ at such relative distances as to produce one'' virtual image at $$RV.$$ It will be noted that the image $$r'$$ is smaller than would have been formed by the objective. The magnifying power of the instrument is therefore less than it would be if the lens $$C$$ were used alone as the eyepiece. This loss of magnifying power is more than counterbalanced by the increased distinctness.

Fig. 135 shows the Ramsden or positive eyepiece. The focal length of the lenses in this combination is generally the same, and the distance between them is two-thirds the focal length. The aid it gives in correcting the residual errors of the objective is evident from the figure.

360. The Rainbow.—The rainbow is due to refraction and dispersion of sunlight by drops of rain. The complete theory of the rainbow is too abstruse to be given here, but a partial explanation may be given. Let $$O,$$ Fig. 136, represent a drop of water, and $$SA$$ the paths of the incident light from the sun. The light enters the drop, suffers refraction on entrance, is reflected from the interior surface near $$B,$$ and emerges near $$C,$$ as a wave of double curvature of which $$mn$$ may be taken as the section. Of this wave the part near $$p,$$ the point of inflection, gives the maximum effect at a distant point, and if the eye be placed in the prolongation of the line $$CE$$ perpendicular to the wave surface, light will be perceived, but at a very little distance above or below $$CE$$ there will be darkness. The direction $$CE$$ is very nearly that of the minimum deviation produced by the drop with one internal