SECOND DEGREE OF CONVENTIONALISM, in which all parts are represented: (c) By a reduction to Monochrome of color, with Gradation (_only_) remaining; (d) By reduction to Monochrome of White and Black, with Gradation (_only_) remaining (see Sec. 16). THIRD DEGREE OF CONVENTIONALISM, in which all parts are represented: (e) By reduction to a Flat-tint of one pigment on a ground of another; (f) By reduction to a Flat-tint of White on Black, or _vice versa_ (see Sec. 17). ULTIMATE CONVENTIONALISM, in which all parts are represented; (g) By reduction to Outline of several pigments; (h) Reduction to Outline of one pigment (see Sec.18). Inasmuch as Realism ceases so soon as any reduction in the three qualities (of color, gradation, and shadow) is introduced; and the treatment becomes more Conventional in each method after the first--these are classified under the heading of CONVENTIONALISM. [There is an analogous scale of reduction in Form, from the Complete-relief of an isolated Statue to the Flatness of a Floor-plate; but this does not belong to the present subject.] * * * * * THE CYCLOSTAT. The various processes commonly employed for the observation of bodies in motion (intermittent light or vision) greatly fatigue the observer, and, as a general thing, give only images, that are difficult to examine. We are going to show how Prof. Marc Thury, upon making researches in a new direction, has succeeded in constructing an apparatus that permits of the continuous observation of a body having a rapid rotary motion. The principle of the method is of extreme simplicity. [Illustration: FIGS. 1, 2, AND 3.--DIAGRAMS EXPLANATORY OF THE PRINCIPLE OF THE CYCLOSTAT.] Let us consider (Fig. 1) a mirror, A B, reflecting an object, C D, and revolving around it: when the mirror will have made a half revolution, the image, C' D', of the object will have made an entire one. The figure represents three successive positions of the mirror, distant by an eighth of a revolution. The structure of the image shows that it has made a quarter revolution in an opposite direction in each of its positions. But if (Fig. 2) the body itself has revolved in the same direction with an angular velocity double that of the mirror, its image will have described a circle in remaining constantly parallel with itself. The image will be just as insensible as the object itself; but it is very easy to bring it back to a state of rest.