A manual of photographic chemistry : including the practice of the collodion process . Blue. Yellow. Bed. A little reflection will serve to show that if, as just stated,the coloured rays are unequally refrangible, white light must,as a consequence, be invariably decomposed on entering anydense medium. This is indeed the case ; but if the surfacesof that medium are parallel to each other, the effect is not seen,because the rays again recombine on their emergence, beingbent to the same extent in the opposite direction. Whenhowever the surfaces are not parallel, but inclined to eachother at an an
A manual of photographic chemistry : including the practice of the collodion process . Blue. Yellow. Bed. A little reflection will serve to show that if, as just stated,the coloured rays are unequally refrangible, white light must,as a consequence, be invariably decomposed on entering anydense medium. This is indeed the case ; but if the surfacesof that medium are parallel to each other, the effect is not seen,because the rays again recombine on their emergence, beingbent to the same extent in the opposite direction. Whenhowever the surfaces are not parallel, but inclined to eachother at an angle, the original divergence being increased, thedecomposition becomes permanentwill show what is meant. The following figures. D. ON THE FOCI OF LENSES. It has already been shown that convex lenses tend to con-dense rays of light and bring them together to a point. Thispoint is termed the focus of the Lens. The folloAving laws as regards the focus may be laid down. 50 ON THE NATURE AND PROPERTIES OF LIGHT. That rays of light which are pursuing a parallel course atthe time they enter the Lens are brought to a focus at a pointnearer to the Lens than others. That divergent rays, on the other hand, do not meet sospeedily,—their focus is longer. Now parallel rays are rays proceeding from distant ob-jects, and diverging rays are rays from objects near athand. The Suns rays are always parallel, and the diver-gence of the others becomes greater as the distance from theLens is less. The focus of a Lens for parallel rays is always termed the principal focus, and it is not subject to variation. Whenthe rays are not parallel, but diverge from a point, then thatpoint is associated with the focus, and the two together ar
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