This morning a pair of clip-on reading glasses arrived at my home from amazon.cm and I was able to make a simple device for imaging the Sun. The lenses are +1.50 diopters and I mounted them on an optical bench to illustrate the relation between the diopter power and the focal length which is 2/3 of a meter.
The small aperture in the index card reduces the brightness of the projected image and improves the sharpness of the image since the lenses were designed to work near the eye and so not all the rays from the glasses will focus correctly on the image card.
The reading glasses do help with working at the computer since I wear bifocals and have some presbyopia. The Visby lenses are evidence that some of my distant ancestors may also have suffered from this condition. These lenses were quite sophisticated for their time.