Page:Journal of the Optical Society of America, volume 30, number 12.pdf/88



24. Change of Color with Change of Particle Size. , E. I. duPont de Nemours and Company.

The previously developed theories of the change in the three attributes of surface color, which accompany a change in size of pigment particles, have been elaborated and corrected, and have been tested by application to a great many cases observed in the laboratory and found in the literature. Excellent agreement of the theory with the facts is found. The chief color changes are: (1) the increase in lightness due to decreased absorption by particles of decreasing size; (2) the dulling due to the greater surface reflection of light not absorbed; and (3) certain hue changes predictable from the changes with increased subdivision of particles, or with decreased thickness of absorbing layer, known to occur in the absorption or transmission curves of solutions of corresponding color. The hue changes are describable as those resulting from the assumption that most of the reflected light from pigment surfaces is due to light transmitted through the particles; and they are in the direction corresponding to decreased absorption in the wave-length region of the characteristic maximum absorption, as the particle size decreases. It is further found that, roughly speaking, subdivision of the particles has the same effect on the hue as admixture of black or white pigment to the chromatic one, or changing from oil film or wet pigment to dry powder in air, except that, especially on addition of certain white pigments, a blue-violet component due to scattering of light by very small particles is blended with the main component of the reflected light.

25. A New Polarimeter Using Sheet Polarizing Elements. , Spencer Lens Company.

A new polarimeter will be described which employs Polaroid material in the polarizer and analyzer. A narrow strip is superimposed on a large disk of this material to form a three-part field in the polarizer. This, like the Lippich half-shade device, can be set to a selected sensitivity and can be used with light of any wave-length. The analyzer contains a disk of Polaroid material which is mounted in a cone bearing and is controlled by a worm gear and worm. The position of the analyzer can be read to 0.1 degree by reference to a graduated drum on the worm shaft. Sample tubes 200 mm long and shorter can be accommodated. In an instrument of this grade, sheet polarizing material offers many advantages. The end point device is simple, easy to mount and to adjust. The glass plates between which the Polaroid sheeting is laminated serve also as splash plates. In the material selected, the polarization is practically complete through the middle of the visible spectrum and is still adequate near the limits of visibility. If one attempts to match the fields when the instrument is illuminated with white light a residual color difference is evident. Because of the rotatory dispersion of the sample it is useless to attempt measurements under these conditions with any polarimeter. When the instrument is used with monochromatic light these color differences disappear. The use of an orange filter to modify the light from a tungsten lamp is common practice with simple polarimeters designed primarily to measure glucose and albumen in urine. In this application the rotatory dispersion has a small effect on the color of the field because the total rotation is so small (5°-10°). Nevertheless it is desirable to supply a filter which will control the wave-length limits of the transmitted band with sufficient accuracy to ensure that measurements made with this illumination will have the same values as reference measurements made with sodium light. A similar problem arises with filters designed to approximate the effect of other wave-lengths. Data on such filters will be presented.