ON  THE  CHANGE OF  REFRANGIBILITY OF LIGHT.
not to disturb the regularity of the refraction. The appearance of the general pencil refracted through a rather large lens, with its caustic surface, its geometrical focus, &c., is singularly beautiful when exhibited in this way, on account of the perfect continuity of the light, and the delicacy with which the different degrees of illumination belonging to different parts of the pencil are represented by the different degrees of brightness of the dispersed light. The solution should be contained in a vessel with plane sides of glass, and ought to be very weak, or else only the part of the pencil which lies near the surface by which the light enters will be properly represented.
Application of internal dispersion to the determination of the absorbing power of media with respect to the invisible rays beyond the violet, and the reflecting power of surfaces with respect to those rays.
194.    Hitherto no method has been known by which the absorbing power of a medium with respect to these rays could be determined for each degree of refrangibility in particular, except that which consists in taking a photographic impression of a pure spectrum, the light forming the spectrum having been transmitted through the substance to be examined.    It is needless to remark how troublesome such a process is when contrasted wifch the mode of determining the absorption which media exercise on the visible rays.    But the phenomenon of internal dispersion furnishes the philosopher, so to speak, with eyes to see the invisible rays, so that the absorbing power of the medium with respect to these rays may be instantly observed.    For this purpose it is sufficient to form a pure spectrum, using instead of a screen a highly sensitive fluid or solid, such as one of those mentioned in Art. 191, and to hold before it the medium to be examined, or else to place the medium over the whole or a part of the slit.
195.    In this way the transparency of glass coloured yellow by oxide of silver with respect to the violet rays and some of those still more refrangible, which has been remarked by Sir John Herschel*, may be at once observed.    A set of green glasses were found to be very variable in the mode in which they absorbed the
* Philosophical Transactions for 1840, p. 39.
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