Motion Picture News (Sept-Oct 1916)

September 30, 1916 ACCESSORY NEWS SECTION The Camera [ilii|i|Pfi|!|i!i!ii||iif[|[!»'![ii A Department Devoted to Motion Picture Photography in All Its Phases, Studio and Laboratory Work Inquiries relative to motion photography promptly ansivered Color Photography {Continued from page 2079) is necessary. In B, Fig. 5, a reflection grating G is shown on the spectroscope table. The collimator tube C is now facing (at an angle) the ruled surface of the grating, while the reflected image of the spectrum is received by the viewing telescope T at the corresponding angle of diffraction. Using the Concave Grating At C, Fig. 5, we see how a spectrum may be produced by the use of the concave grating, which latter, as was stated last week, was one of Rowland's most valuable discoveries. With the concave grating no collimator lens or telescope is necessary. The light ray from the slit S is reflected from the concave grating G and forms a diffraction spectrum R V which comes to a focus (without the aid of lenses) on the circumference of a circle having as its diameter the radius of curvature of the concave 1 II 1 \ B. JITa.. c. Zz. 2. II ill nil II grating. As was previously stated the spectra produced by concave reflection gratings possess elegant definition and the concave grating has come into extensive use. Before leaving the subject of diffraction spectroscopes we must explain how a grating spectrum may be examined at direct vision, e. g., in a straight line with the slit and collimator. For this purpose we include a diagram of a pocket diffraction spectroscope, D, Fig. 5. In D, Fig. 5, A is a tube having at one end the slit B. Arranged to telescope, or slide, within the tube A is another tube C having at one end the collimator lens D and at the other end the eyepiece or viewing aperture F. Between the lens D and the eyepiece F is a prism E of small angle (about 20°) to which a celluloid grating replica is cemented. The narrow angle prism serves to throw the central white image of the slit to one side, so that nothing but the spectrum is seen by the eye. While a pocket diffraction spectroscope will show more of the Frauenhofer lines than does a pocket prism spectroscope, and while it also has the advantage over the prism instrument in its correct distribution of the colors, the same remarks regarding its inadequacy for exact measurements apply to it as to the pocket prismatic instrument described two weeks ago. As a matter of interest we might state, however, that we have, with a high grade pocket diffraction spectroscope of English make, been able to observe the D line just barely separated, by directing the instrument toward a strong beam of sunlight from a Heliostat. As we have given in this and the previous installments of our article " Color Photography," a rather extended description of the various forms of spectroscopes, we will now outline a few of the many important uses of such instruments in everyday photographic practice, and in the photographic analysis of color. Uses of Spectroscope in Photography With the spectroscope a safe-light for the dark room may be examined and its suitability readily determined. Likewise — a good estimate may be formed of the quality of illumination produced by any of the various arcs or other forms of studio lights, such as are used in film production. But in the field of color photography (and cinematography) and the photography of objects in thein correct luminosity values (orthochromatic photography) the spectroscope plays a vitally important role. It is with the spectroscope that the suitability of the dyes used in sensitizing emulsions for color work is determined, and the adjustment of suitable light filters to color sensitive emulsions could never be accomplished with scientific accuracy vnthout recourse to spectroscopic tests. In addition to the study of various light sources, such as the solar spectrum and the spectra of arcs, there are two other classes of spectra which must receive consideration by the student in color photography. Emission Spectra The first of these which we will mention is the emission spectra of colored flames. As is known to all students of elementary chemistry, a piece of iron or platinum wire, when dipped into a solution of a metallic salt and then held in the flame of a Bunsen burner, will impart to the Bunsen flame vivid color. These flame colors differ for different metals and when the Bunsen flame, colored by the combustion of a metallic salt, is examined with the spectroscope, a line spectrum is seen instead of the customary band of spectrum colors. The line spectra of many elements correspond exactly, as to location, with well known fixed lines in the solar spectrum and like the fixed lines of these various elements also appear at fixed and unvarying places in the spectrum. A, Fig. 6, is a comparison scale showing some of the more prominent lines in the solar spectrum. The line spectrum of Sodium is shown at B in Fig. 6, where it will be noted that the yellow Sodium lines appear in exactly the same position in the flame spectrum as do the fixed D lines in the solar spectrum. Another prominent line spectrum is that of Lithium shown at C, Fig. 6. The line spectrum of Lithium shows a beautiful red line between the B and C lines of the solar spectrum and an orange line nearer to the D lines. When studying the spectra of the Bunsen flame colored by the various metallic salts nothing but the characteristic bright lines are seen through the spectroscope, because the Bunsen flame is not luminous enough to produce an easily visible complete spectrum. In the course of our article we will again refer to these line spectra, and explain their use in photo-spectroscopic work. Even more important than line spectra in the photographic conquest of color are the absorption spectra of the colored solutions of chemicals and dye stuffs. Absorption Spectra When a solution of a dye or other liquid, in a glass cell, is placed between the light source and the slit of the spectroscope