After taking the measurement a b and at the moment when the hand is raised for comparison, if the observer's hand is closer to their eye, the result of the comparison would no longer be accurate. Indeed, a b would be two-thirds of e f, while D E is a third of B C, yet a c and e f equally represent, to observer A, the height B C. If, instead of bringing the hand closer, one moves it away, it is clear a similar issue would arise.

We must assume that it is understood now the necessity of keeping the hand always at the same distance from the eye to compare the sizes of objects. No matter where you initially place your hand. To compare, say B K to B C, it doesn't matter if observer A, for convenience, places the hand at point m or point n, as long as the hand then precisely descends along the vertical m p or n s.

We have often encountered people who, not understanding well at first, after finding the first measure, would immediately transfer this measure onto paper instead of comparing it, as if observer A transferred the height ab onto paper, instead of making the comparison as previously explained. But after you have marked this line on the paper, who tells you that you will replicate your hand at the exact same distance from your eye as it was before, to continue measuring the object's dimensions and compare them amongst themselves? This method would only be good if both the hand and the eye had a fixed support point.

It is well understood however that you are only obliged to keep the hand at a fixed distance during the exact time required for the comparison which is to give you one of the dimensions. When it concerns finding one of the other dimensions, it won't matter the distance at which you place your hand, to make this new comparison, as long as you keep the hand at the same fixed distance throughout the duration of this new comparison.

A simple method to simultaneously measure the compared sizes of various dimensions of objects one wishes to draw from nature.

You can simultaneously measure the comparative sizes of the various dimensions of any object by proceeding as follows:

Take a strip of paper fixed on a ruler or stick. Hold this stick at any distance from your eye, in a vertical position if you wish to compare heights, or horizontal if it concerns widths. Suppose it is vertical. This strip of paper is held perpendicular in front of the eye, and with the other hand, you mark on the paper, using a vernier, points corresponding to the object's extreme points. You will mark in the same manner the dimensions of the object’s various parts or details. This operation should be completed immediately, without moving the hand or eye.

As an example, observer A (plate 2, fig. 3), who wants to find the simultaneous comparative heights of the various parts of column BC. He will hold vertically, at a certain distance from his eye, a strip of paper, m p, for instance. He will mark on this strip the points m, a, b, l, p, representing on his eye B, K, E, D.

Another example. Observer A (fig. 4), in the same way, will find, on the paper ruler MP, the different proportions of figure BC. If he takes the measure in EF, closer to his eye, the figure will be smaller, but the proportions will be the same between them.

Explanation of two frames frequently used in drawing from nature, the so-called geometrical plan, and the so-called perspective plan.

In the preliminary notions of Geometry, it was seen what a plan is. It is evident that all the objects to be drawn present themselves to our view terminated by faces that are plans. Before starting to draw from nature, it is essential to know two kinds of plans, which will appear to our eyes, at each instant, in practice.