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 crime of scribbling with pencil or ink on the printed pages. A thread of white or black silk or cotton laid upon the surface will serve your purpose.

To explain more clearly. Lay a thread of cotton on one of the perspective diagrams, hold one end on the vanishing point, and from that angle swivel the thread on to the various parallel lines. Remove the thread to each vanishing point.

Test any of the perspective examples in this way by merely laying the thread on the paper, holding the right thumb on the thread at the vanishing point. An old reel with a small quantity of cotton is, perhaps, easiest to handle, then the thread does not slip out of the left hand.

Use black thread if the drawing is lightly sketched on white paper, and white thread if the picture is in a dark tone.

Let me presume that you wish to analyse the perspective of the accompanying photograph of a picture-gallery, which is a very simple example.

Lay the thread first against the lowest line of the left wall, and find the inclination of the floor; then lay it on the top line near the ceiling. You will easily discover the point where these two lines meet. Hold the thread on that place and test the right-hand side of the picture by laying the thread first against the base and then against the summit of the pillars.

By careful adjustment you will soon fix the actual position of the vanishing point, which lies, does it not, between the two dark frames on the facing (far) wall and the light frames of the two adjoining pictures.

All the pictures on the left-hand wall lie parallel with the wall and diminish as they recede. All the pillars on the right side diminish both in height and bulk. Is not the nearer pillar a great deal larger in girth than the next, and the second pillar larger than the third?

Although the pictures are grouped at different heights from the ground, yet they all diminish to the same vanishing