COLOUR ABSORPTION Whilst colour itself is an absorption of light (see " Colour "), it is extremely important in some cases to know the colours absorbed by certain materials, such as aniline dyes for filter making. The only method of determining this satisfactorily is by means of a spectroscope, or, for accurate work, a spectrophotometer. It is laborious work, as the absorption of a dye solu tion will alter with increased concentration or depth of solution, and it is necessary, therefore, to make very careful spectro-photometric obser vations at various dilutions. This, however, can be performed much more readily by, hotography, as has been done by Uhler and Wood, of the Carnegie University, of Washington, U.S.A., and more completely by Wratten and Wainwright in their " Atlas of Absorption Spectra," which con tains the absorption spectra of 17o dyes. For this work was used a small box spectrograph fitted with a prism grating, and the dye solution was contained in a wedge cell of rectangular form of r cm. (.4 in.) internal length and 5 mm.
internal width, with a diagonal partition which divided it into two wedge-shaped cells, the one being filled with the dye solution and the other with the solvent, so as to obviate the prismatic effect of the cell. The thickness of the dye solution thus varied considerably, the actual thickness from end to end of the slit being about r to 15. The spectrograph was provided with a wave length scale and an ultra-violet filter. Pre cisely the same results can be obtained by using a parallel-sided cell of fixed width and varying the strength of the solution, or keeping the dye strength constant and varying the cell width, but these plans are laborious and do not give the required information in such compact form.