Protector scopes

[BCA]

I recall from my work with rock thin sections that Canada Balsam was used because the index of refraction was the same as glass. Differences in the index of refraction between different layers of glass or other substances would cause optical problems.
I'm pretty sure that the M3 protectoscopes look the same as the ones for the M5 Stuart and M20/M8 armored cars but I don't have a Stuart M3 book to confirm: Gina: Do you have a part number from your M3 book.
Finally, it may be that much of the periscope effect is from internal reflection within the prism, not from mirrored surfaces.