equal to ml, and you find the point f, corresponding to the point F of one of the corners of the model box.

By a similar operation, you will find the angle e. The points f and e which represent the angles F and E being thus found, draw the line ef which represents the line EF, and the lines df and what represents DE and CE and thus we have obtained the face cde, which represents the perspective plan forming the top of your model box.

It may also be helpful to repeat the simple thread experiment that we indicated earlier. Attach threads to each corner of the model box, at angles E, F (fig. 6); bring the two ends of these threads close to your eye, and you will see that the thread starting from angle F will not pass over angle D, but instead will intersect side CD at the same point P that you found by the previous operation. The same result would be found for the other angle E.

Note on using the pencil holder as a plumb line.

When you hold the pencil holder upright, to see at which point the perpendicular dropped from angle F will intersect CD, this pencil holder being quite close to your eye, may hide, by its thickness, a large part of the box, and you won't know exactly where the vertical dropped from point F intersects line CD. To be more accurate, one should search for the plumb line with a fine thread. It is ordered very thin at the end of which a small weight is placed, as seen in the figure. However, you can stick to the pencil holder, being careful to take the plumb line on one of the sides of the pencil holder, either to the right or to the left.

Drawing from nature a box seen from the side.

Now let's take for model a box, still rectangular, but placed in such a way as to be seen from the side, as shown in figure 10.

We still assume the front ACDB of the box placed on a line parallel to the observer; this plane remains geometrical. The top of the box, CDFH, is, as in the previous operation, a perspective plane. The lateral face BDFE, although vertical, having two vanishing lines BE and DF, is a second perspective plane.

We will start by constructing the rectangle of the geometrical plan ABDC, as we constructed that of the previous operation (fig. 6 and 7). Then, while searching for the top of the box, we will deal with the lateral face BDFE.

The line BE, although placed on a horizontal plane, seems, because it moves away, to rise in front of the observer. The angle E will appear higher than angle B. It is necessary to determine how much higher angle E appears than angle B. The method you are going to use is the same as for previous applications. With one hand or both hands, you hold the pencil holder in horizontal position (fig. 11), raising or lowering it until it hides angle E from your eye. The pencil holder, at the same time as it hides this angle, will intersect line BD (fig. 10) at any point, which will indicate the height of angle E. Here, the intersection will occur at point g, mid-point of BD. You mark a point on your drawing that divides the line representing BD into two equal parts, and from that point, you draw a horizontal line.

It is necessary to now find what the distance is, as you see it, from line BD to line EF. You hold the pencil holder horizontally and always parallel to yourself, and place it in such a way that the end by the hidden side of your thumb to your eye places a point on line BD. You position your index finger so that it is level with point E of angle E. This is exactly the same operation as in plate 2, fig. 3, with the sole difference that the pencil holder, then vertical, is held here in a horizontal position. You thus find on the pencil holder the distance g E, which you compare, always by eye and on the pencil holder, either to distance g F, or to distance FE.