SURFACTANT INTERACTIONS 267 As in the previous work (1), for all light-scattering experiments, polarizers having directions of polarization perpendicular to the plane of incidence were placed between the light source and the sample and between the sample and the light detector of the goniophotometer. In the current experiments, a Glan-Thompson prism was used in front of the sample, while a sheet polarizer was placed before the light detector. Figure la shows the bottom plate of the GP-1R sample holder used for mounting hair fibers. The diameter of the opening in this plate is 7.3 cm, while the raised plastic ring is 2-mm high and 2-mm thick. In order to accommodate up to 40 hair fibers, the plastic ring contains 40 grooves, 0.635 mm apart, cut at opposite ends of the ring. In all experiments in this work, only one hair fiber was used per scan. These fibers were mounted on the bottom plate under the constant tension supplied by a 2-g alligator clip and held in place by a cover plate secured with two strips of magnetic tape. Figure lb shows the complete sample holder with a single fiber mounted. All tress measurements were made by taking 21 hairs from a treated tress, scanning Figure l a. Photograph of the bottom plate of a GP-1R hair sample holder. Note the four circular magnets set in the bottom plate and the two holes on either side. These are used to mount the sample holder on the vertical mount in the light-scattering instrument.

268 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure lb. Photograph of hair sample holder with one hair mounted and top plate in place. Note that the holder is mounted in the GP-1R with the hair fiber in a horizontal position. Also, because of the magnetic tape on the bottom plate, the use of the four fasteners is optional. them one at a time, as stated above, from 0 ø to 75 ø, and averaging the results. The time for a single scan was 75 seconds. All SAC experiments were run with the hair fibers at an incident angle of 30 ø and oriented so that the direction of incident light was toward the tip of the hair [RER orientation (3)]. All experiments involving commercial conditioner were run at an incident angle of 45 ø in order to compare the results to previous measurements run on the Brice-Phoenix photometer, an instrument on which scans from 0 ø to 75 ø could only be run at 45 ø incidence. In the Brice-Phoenix experiments, for an RER orientation at 45 ø incidence, reflection from the back walls of hair fibers was found to interfere with the estimate of diffuse scattering at 75 ø (1,3). All the former 45 ø measurements on the Brice-Phoenix instru- ment, and also the current GP-1R 45 ø measurements, were therefore run with the hair fibers in an REL orientation.