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Bausch & Lomb Optical Company the iris diaphragm, which limits the area on the screen when projecting comple- mentary colors; the first lens which images the diaphragm upon the screen and also forms a small image of the spectrum in the plane of the slit on the vertical stage; vertical stage with horizontal slit and clips at the back for holding small refracting wedges; the second lens in hinged mount which projects a magnified image of the spectrum on the screen. For projecting complementary colors the vertical stage, Figure 2, has a verti- cal and cross slide with clips for holding small deflecting prisms, of which three are supplied of varying widths. By placing one of the deflecting prisms on this slide and moving it to any desired point a portion of the spectrum will be deflected, and there will be two images of the opening of the iris diaphragm upon the screen. One will represent that portion of the spectrum deflected by the prism, and the other will represent the remainder of the spectrum which passes through to the screen without refraction. Where the two circles overlap there will be clear, white light. This prism can be moved horizontally through the entire spectrum, deflect- ing any portion that may be desired. By putting in two prisms with the bases in opposite directions—that is, one facing downward and the other upward—three circles can be seen upon the screen. When showing complementary colors, the lens at the end of the supplement- ary bed should be turned out of the optical axis, the lens mount being hinged for this purpose. By turning this lens into the optical axis one can determine exactly what portion of the spectrum will be deflected by the prism. If one wishes to project absorption and emission spectra only, this can be done with the adjustable slit and the bottle prism, as indicated in the price list. Special Carbon Holders for Projecting Spectra of Chemical Salts The spectra of chemical salts are projected by intro- ducing the salt into the arc. If a pure spectrum is to be obtained the carbons must be changed with each new salt, but as this cannot be done conveniently with the ordinary lamp, we have de- vised a pair of Figure 2— Arc Lamp Showing Revolving Electrode Plates. special carbon holders with revolving electrode plates, which are attached to the lamp, as illustrated in Figure 2. There are five pairs of electrodes and the lower carbons are fitted with quartz cups in which the salts are placed. Thus a different cup is provided for each salt and the upper carbon changed, as well. The vapor of the salt becomes the light source and the characteristic spectrum, just as seen in the spectroscope, is projected on the screen. 120