The text discusses the importance of carefully determining the opening of a convex glass lens in drawing. It suggests that the lens should typically be set as one would for telescopic lenses, adjusting for lighting conditions to achieve better line definition. It highlights the balance between preventing excess light reduction while maintaining precision in depicting objects.
Method for Learning
...marked as desired, since at all the distances one might choose, there will always be many rays whose convergence occurs more than an inch before or beyond the paper. The confusion arising from this diversity, although not very noticeable to the eye, still causes harm and prevents an exact resemblance from being captured. I make this note here to give a fair idea of the value of this machine, also highlighting in what way it might be genuinely useful and how its apparent utility is susceptible to errors that experience reveals more than reasoning.
VIII.
On the opening of the convex glass.
27. In all previous articles, one should not neglect to examine the opening to be given to the convex glass: because although one cannot reduce this opening to a fixed measure, it is always good to heed the following remarks.
1°. One can usually give the glass the same opening as one would give to a telescope whose lens is this glass’s objective.
2°. This opening should be decreased when the objects are very illuminated and increased when they are exposed to weaker light.
3°. Lines appear more precisely marked with a smaller opening than with a larger one; therefore, when drawing, one should give the glass the minimal opening possible: however, it is essential to not overly diminish the light that only enters through the machine.
Translation Notes: The term "convex glass" refers to a lens used in drawing aids of the period, akin to those in optical devices like telescopes (referred to as 'lunette d'approche'). These tools aided artists in achieving accurate proportions and light rendering.