The microscope and its revelations . on. It is this difference in theirrefrangibility, which causes their complete separation by theprism into a spectrum; and it manifests itself, though in aless degree, in the image formed by a convex lens. Eor ifparallel rays of white light fall upon a convex surface, themost refrangible of its component rays, namely, the violet, wiUbe brought to a focus at a point somewhat nearer to the lensthan the principal focus, which is the mean of the whole; andthe converse wiU be true of the red rays, which are the leastrefrangible, and whose focus will therefore be
The microscope and its revelations . on. It is this difference in theirrefrangibility, which causes their complete separation by theprism into a spectrum; and it manifests itself, though in aless degree, in the image formed by a convex lens. Eor ifparallel rays of white light fall upon a convex surface, themost refrangible of its component rays, namely, the violet, wiUbe brought to a focus at a point somewhat nearer to the lensthan the principal focus, which is the mean of the whole; andthe converse wiU be true of the red rays, which are the leastrefrangible, and whose focus will therefore be more distant. * It has been deemed better to adhere to the ordinary phraseology, whenspeaking of this fact, as more generally intelligible than the language inwhich it might be more scientifically described, and at the same time leadingto no practical error. 48 OPTICAL PRINCIPLES OF THE MICROSCOPE. Tims ill Fig. 7, tte rays of white light, a b, a b, which fall onthe peripheral portion of the lens, are so far decomposed, that Fig. Diagram illustrating Chromatic Aberration. the violet rays are brought to a focus at c, and crossing there,diverge again and pass-on towards e f. On the other hand,the red rays are not brought to a focus until d, and cross thediverging violet rays at e e. The foci of the intermediaterays of the spectrum (indigo, blue, green, yellow, and orange)are intermediate between these two extremes. If the imagebe received upon a screen placed at c, the focus of the violetrays, violet will predominate in its own colour, and it will besurrounded by a prismatic fringe in which blue, green, yellow,orange, and red may be successively distinguished. If, on theother hand, the screen be placed at d, the focus of the redrays, the image will have a predominantly-red tint, and willbe surrounded by a series of coloured fringes in invertedorder, formed by the other rays of the spectrum, which havemet and crossed.* The line e e, which joins the points ofintersection betwee
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