. Public health laboratory work, including methods employed in bacteriological research, with special reference to the examination of air, water and food contributed . ent a short intro-ductory description is appended :— If a compound light, such as sunlight, is made topass through a glass prism, the different coloured raysof which it consists are unequally refracted, (or bentout of their original course), so that beyond the prismthey form, upon a white surface, a continuous line ofcolours called the spectrum; and the spectrum of thecompound white light will be seen to consist, in order,of red
. Public health laboratory work, including methods employed in bacteriological research, with special reference to the examination of air, water and food contributed . ent a short intro-ductory description is appended :— If a compound light, such as sunlight, is made topass through a glass prism, the different coloured raysof which it consists are unequally refracted, (or bentout of their original course), so that beyond the prismthey form, upon a white surface, a continuous line ofcolours called the spectrum; and the spectrum of thecompound white light will be seen to consist, in order,of red, orange, yellow, green, blue, indigo, and violet. CARBONIC OXIDE. 213 A number of dark lines—called absorption bands, orFraunhofers lines—are also seen to cross the image ofthe solar spectrum. In other lights, however, the spectrum will only showa very few bright bands (that of the sodium flame onlyone), and the remainder of the spectral image is thusrendered almost—or quite—invisible, by comparison. If now we transmit solar light through differentcoloured solutions, we then get different absorptionbands—though analogous to the lines of Fig. 34.—The spectroscope. If a solution of fresh blood, for instance, be taken, andthe cell containing it is placed before the slit in theinstrument which admits the light, two distinct andcharacteristic absorption bands are seen. The accompanying figure will serve to show themanner in which the spectroscope is constructed :— A firm iron stand is seen to support, at its upperend, a brass plate carrying the glass prism ; laterallya cylinder is also fastened to the brass plate, and inthe end of this cylinder which is nearest the prism isplaced a lens, the other end being closed by a plate 214 LABORATORY WORK. with a vertical slit in it (the breadth of which can beregulated by a screw to meet requirements), and throughwhich slit the light (represented by two coloured flamesin the figure) is admitted to the prism—the
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