684 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS experienced during installation, but these were attributable to the rigours of a new application. Some further trouble was experienced approximately 24 months later, when a new television camera was installed. The old camera tube (1 700 h old) had not failed, but i•nage quality had deterior- ated {both resolution and contrast). Errors arising from the sample The nature of the sample will have a considerable bearing upon the ease and accuracy with which it is evaluated. The predominant problems of discrimination have already been discussed in detail elsewhere (9-11). Other aspects, such as the opacity of the sample and the presence of dust, have also been described (13). The way in which the sample is representative of the material from which it was withdrawn is of paramount importance. To reduce the magni- tude of this error, more features must be examined, and they must be withdrawn from a greater proportion of the original. It is regrettable that on many occasions much emphasis is placed upon the exact evaluation of a sample without due regard for the accuracy with which it represents the whole (an excellent example is the conclusion drawn from single optical and electron photomicrographs). The magnitude of errors arising from unrepresentative sampling have already been examined (13, 14). SAMPLING AND THE PREPARATION OF SAMPLES The necessity for representative sampling is accepted by anyone in- volved in particle size analysis. If we are going to use an automatic optical means to achieve this end, additional conditions must be imposed. To avoid difficulties due to the inability of the computer logic to separate touching or overlapping particles from re-entrant surfaces, the particles must be well separated, and as regular in shape as possible. However, in attempting to facilitate the separation, or distribution, of particles, care must be exercised so as to avoid the dispersion of aggregates and agglomerates. The extent to which this is a problem will depend upon the type of sample being examined powders obviously give considerable difficulty as do liquid suspensions, but the problem does not arise when the particles are sus- pended in a solid matrix. For these reasons the type of specimen prepared for evaluation will vary, according to the morphology of the sample.

THE PARTICLE SIZE ANALYSIS OF PIGMENTS WITH THE QUANTIMET 685 Powder methods There are several methods by means of which a sample of powder may be dispersed, and mounted, to provide a suitable specimen. These include dispersion in a liquid of low boiling point. After spreading the suspension out onto, for example, a glass slide, the liquid evaporates leaving the powder behind. This is generally unsatisfactory due to the extensive flocculation/agglomeration caused by the evaporating liquid (the resulting surface tension draws the small particles into the vicinity of the large ones). The situation is only partly remedied by the inclusion of a binder such as collodion (15). Alternatively, the powder may be dispersed into a high boiling point liquid by gentle mixing. The liquid suspension is diluted if necessary to give a reasonable particle concentration on the microscope slide, and mounted under a cover glass. When a powder is relatively coarse, e.g. 50 pm and above, it can some- times be advantageous to dust the sample onto a glass slide. This does, however, introduce the possibility of errors due to fractionation with the finer portion of the size range being lost. However, none of these methods is completely satisfactory (16). Liquid suspensions These have already been described under the previous heading. The liquids are of relatively high boiling point, and the final concentrations must be kept sufficiently low to prevent overcrowding on the microscope slide. Solid suspensions From most points of view this is the most convenient form in which to handle pigment particle populations. The material may be easily sampled by taking thin sections from a convenient moulding, or alternatively the material may be extruded as thin film the population is not subjected to size fractionation, since all particles are retained irrespective of size changes in particle size distribution cannot take place the samples are easily obtained and handled. A number of strips of film, or microtomed thin sections are examined using one of several available magnifications. The samples are selected from various parts of the specimen, and several areas examined from each sample.