  When Descartes speaks of taking into account  the “size of the images” on the retina, he need be speaking only of visual angle, which can be taken as equivalent to retinal-image size. Also, in saying an object ten times farther away than a near object should be a hundred times smaller, he is speaking of area; it would be ten times smaller in linear height. Accordingly, Descartes here describes a process in which visual angle is combined with perceived distance in order to yield a perceived “size at a distance”  that reflects the object's constant true size in those cases in which we correctly register and combine distance with visual angle.