. Light, a textbook for students who have had one year of physics. in manipulation. Forthe theory of the concave grating, the student should consultsome more advanced text, such as Prestons Theory of Light,or Balys Spectroscopy, where the following relations areproved : Suppose that, in a plane through the middle of the gratingand perpendicular to the rulings, a circle be drawn whosediameter is the radius of curvature of the concave surface, sothat the circle is tangent to that surface at one end of thediameter, and passes through the center of curvature at theother end. Then, if, the slit be
. Light, a textbook for students who have had one year of physics. in manipulation. Forthe theory of the concave grating, the student should consultsome more advanced text, such as Prestons Theory of Light,or Balys Spectroscopy, where the following relations areproved : Suppose that, in a plane through the middle of the gratingand perpendicular to the rulings, a circle be drawn whosediameter is the radius of curvature of the concave surface, sothat the circle is tangent to that surface at one end of thediameter, and passes through the center of curvature at theother end. Then, if, the slit be placed anywhere on this circle,the different spectra will be sharply focussed at various pointsabout the same circle. Moving the slit will of course cause thespectra to shift their position, but so long as the former 130 LIGHT remains on the circle, the latter will also. Rowland adopted avery simple and ingenious method for making sure that theslit remains on this focal circle, as shown by diagram in figure70. Two beams, AS and BS, are set up making an angle of. Figure 70 90°. The slit is placed at the intersection of these two beams,i. e., at S. The grating G, and a plateholder P, to hold a longnarrow photographic plate, are mounted at opposite ends of athird beam, whose length is equal to the radius of curvatureof the grating. Thus, the center of curvature of the gratingcomes just at the middle of the photographic plate. The endsof the beam GP are mounted on slides or carriages, so thatthe end G can slide along the beam BS while the end P slidesalong the beam AS. This beam therefore. always forms thehypothenuse of a right-angled triangle, the acute angles ofwhich can be changed. From the geometrical proposition thatan angle inscribed in a semi-circle is a right-angle, it followsat once that S always remains on a circle of which GP is adiameter. Of course, the photographic plate P is bent to fitthis circle. 64. The ultraviolet region. Fluorescence. Phosphores-cence. Photograp
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectlight, bookyear1921
