I have to say I am not a fan of the clear periscopes or vision blocks provided in this kit and other kits. To me, they are gimmicky. The detail is extremely hard to make out with the transparency and a hassle to mask during painting. Realistically the clear glass look is not even appropriate for modern AFVs.
One of the things I've done with vision blocks in transparent plastic is to start by dipping them in Future to enhance their transparency. then paint the non-optical surfaces silver, followed by black (or whatever color the frames were actually painted). The final touch is transparent green or purple over the optical surfaces. It still doesn't do a good job of replicating modern protective coatings on optics, and I agree that having the pieces in transparent plastic is sort of pointless given the coatings, but it does a decent job for WWII armor vision blocks.