Ok. Let me be more detailed. Usually the wooden frame where the ground glass go, has shoulders from four sides on which the ground glass slipping. The Shen-Hao designed the way that just two shoulders from two opposite sides of the frame supporting the ground glass. The other two shoulders from other two opposite sides of the wooden frame a made dipper, and there is a gap approximately 2mm. between the shoulder and the ground glass. These two shoulders made dipper in purpose to support the Fresnel. The Fresnel itself, if it is made especially for the Shen –Hao has fickness and cut to the size to slip on these two deeper shoulders, and not on the other two witch supporting the ground glass. This way when the Fresnel inserted, the ground glass stays on the same position as without Fresnel.