Other academics have argued that the art of making
• roidi de botacelis et da ogli (disks for vials and for eyes) that could be used to correct presbyopia when held before the eyes and
• lapides ad legendum (stones for reading and for magnifying lenses)
This record provides the earliest written distinction between magnifying lenses which had already been in use for centuries by that time and the new invention of eyeglasses. Had the record been dated before 1286 it would have provided irrefutable evidence pointing to Venice as the birthplace of eyeglasses. The following year, in 1301, an additional provision allowed the manufacture and sale of of vitreos ab oculis ad legendum (glasses for eyes and for reading) by anyone subject to an oath binding the seller to label them as spectacles with glass lenses.
While the above regulations give information regarding the manufacture and sale of glasses, they reveal nothing about the shape of the lenses, convex or concave. Since no other evidence exists for concave lenses for correction of myopia until the fifteenth century, we assume that the lenses must have been convex.
It is fascinating to see that these frescoes display the concomitant use of convex lenses and concave mirrors as magnifiers as part of the scholars equipment. It is obvious that Tomaso and the Dominicans were familiar with the use of these tools and would not have wished to convey the impression of novelty in the artistic depictions.
Medieval Optical Theory:
The curious might well ask what (if any) optical theory underpinned the invention of glasses? The answer to that prescient question is that most scholars agree that the discovery was probably made more through accident than design, most probably by a glass maker, since medieval optical theory could not have led to the invention of glasses. This is because medieval optical theory was based upon the invalid premise that the laws of refraction applied only to a single lens or refracting surface and that the seat of vision lay not at the retina, but at the front side of the lenticular crystalline lens of the eye. Rays of light ('vision') would according to theory be refracted on its posterior side. By such theory, placing a lens before the eye should have caused a double refraction. While corrective lenses seemed to work in practice, they did not make sense in theory. Three centuries would pass before the momentous discovery of the true fundamentals by Johannes Kepler. In this intervening period the evidence shows that eyeglasses were used at first to correct presbyopia and later myopia without any recourse whatsoever to optical theory.
In conclusion, while the use of lenses for magnification is well documented in the ancient world, the existence of spectacles cannot be attested with the same degree of certitude. It has been speculated that glasses were not invented in the great cultures of antiquity (Rome, Greece or Egypt) because few lived into their forties and this in turn meant that few would have suffered from presbyopia. Thus the demand for spectacles to correct this condition was probably small. By contrast, longer life expectancy and the intensely commercial society of late medieval Italy with its developed crystal and glass industry, where merchants and artisans wrote detailed accounts and correspondence known as the mercantesa, combined with the vigorous intellectual life of monasteries and universities ensured the enormous demand for visual aids of all kinds.