The supports themselves move round on an axle fixed on the rule. At the extremity E’ of this rule is fixed a copper blade curved in E’, C’, and pierced by a small hole of about an eighth of an inch in circumference, the edges are made thinner as represented in the diagram placed above; the widened part is turned toward the frame. So much for the body of the instrument, now for the accessories. In the slot left in side, A B C D, lower a glass pane covering the space, _a, b, c, d_, which is not, however, a necessary condition, it depends on the size of the design you desire to take. This pane requires a little preparation. It is done in this way: One chooses a pane of the desired glass, as free as possible from defects. Cover one side only with turpentine, and which you know is a natural varnish. See that this coating is as thin as possible, and to ensure this, go over the surface with a very soft brush steeped in the liquid. When you see that the latter does not run any more, leave off brushing. Let it dry for two days if necessary; take care, meanwhile, to protect the varnished side from dust. Now it remains only to show the use of the instrument. Put yourself in front of the object you wish to represent. Put the frame in a perfectly horizontal position, slide the pane in it, and dispose the rule, E E’ (Fig. 2), in such a manner that, when looking through the little hole, O, you are able to see the object you want to draw. [Illustration: Fig. 2.] Then, with a blue or other colored pencil, trace the outlines of the object on the glass coated with turpentine, the use of the latter being to allow the pencil marks to fix itself on the surface. One sees that the outlines thus obtained will be those of the real object as clearly as possible because they are traced as seen, so to speak. But the principal object of the instrument is not so much exactness of outline as to get the exact proportion existing between the different sizes of the objects placed in different planes. We will try to show this last result by means of another figure. Suppose A B to be an object situated at a certain distance from the eye posted at _o_, the rays from the eye, O A O B, meets the instrument at _a_ and _b_, and the image of this object is given by the line, _a b_. Now, suppose A’ B’ to be another object situated beyond A B; the eye has not changed position, it cannot do so, with reference to the glass, on account of the small rule which is fixed; the image of the object A’ B’, will be _a’ b’_; thus, one has the true dimensions of A’ B’, in respect to A B. It is precisely this proportion which must exist between the sizes of the objects placed in different planes, which constitute perspective. The instrument, therefore, well deserves its name of Perspectograph.