These prizes are based on the lens properties of the optic glass sphere, it´s proportions came out from the focal length of the sphere as well. The circular stainless steel metal ribs that are surrounding the sphere in 3 axes have edges exactly in the focal length, which is 1/4 of the diameter of the sphere. In this case it is 2cm for 8 cm sphere diameter and it is the same proportion used for annulus of the rib. This structure is necessary to prevent spontaneous ignition of the flamable material caused by sun. This design came out from the use of this phenomena in  the meteorological equipment called heliograph, which records sun shine activity during the day on a heat sensitive stripe.