The scientific procedures with which we are concerned here fall into two main groups. Of these, one includes various techniques for extending the range of human vision. The simplest is examination by microscope, which enables characteristics of a surface to be seen that would otherwise be invisible, so that, for example, painted cracks or cracks artificially induced can be distinguished from crackle due to age. With the microscope, too, evidence of removals and additions can be obtained, such as the manipulation of signatures and inscriptions, or the presence of repaint or artificial patina; while the structure of pigments, stone, etc., can be ascertained, as a step toward their identification. More elaborate is examination under various rays of the spectrum, to which the human eye is not sensitive but whose results can be recorded. The best known of these is X-ray, which penetrates certain substances but is held up in different degrees by others, especially metals, so that a photographic film behind an object will record a map of such substances in an object, thus revealing much that is below the surface. On the other hand, ultra-violet rays falling on a surface cause fluorescence, which varies according to substance and texture, so that additions to the surface may be revealed. Infra-red rays, in contrast, penetrate the surface, and are reflected back from the layers beneath, so that a photograph taken by infra-red light may reveal something concealed from the eye, which X-ray may not pick up. The second group of investigatory methods includes various means of analyzing the materials present in an object. The most familiar is chemical analysis; but this is being supplemented and to some extent displaced by spectrographic analysis, with its recent extension in the use of X-ray diffraction. By these means, it is possible to detect even minute traces of substances whose presence or absence may be decisive in settling the date or provenance of a material. Some recent applications of quantitative analysis have proved helpful in ascertaining the date of objects. One of these techniques, determination of the extent of fluorination, was used to prove that the jawbone of the Piltdown skull was a modern forgery; while another, based on the amount of radio-active carbon present, which is known to decay at a certain rate, is still in course of development, but promises to be most useful.