But if you wish to assure yourself in a concrete way that the image of objects recedes from the lens when the objects approach it, and that it approaches the lens when they recede from it, you can use the artificial eye, whose tube I assume can, by being lengthened or shortened, approach or recede from the objective lens, the mat glass, where the images of external objects are depicted; and the change of location of these images will be all the more perceptible as the lens is less convex.