Note. We have found that with the glass blocks commonly available in the UK, the angle between one or both of the two possible incident faces and the parallel faces at which internal reflections occur can be in error by up to 1.5°.This results in asymmetry in the observed angles of incidence required to bring the emerging ray to the midpoint at the end of the block, and complicates interpretation of the results. It is therefore advisable to check the accuracy of alignment of both ends of all glass blocks, for example by standing
the blocks upright on a flat surface alongside a set square. On testing 20 glass blocks in this way, 15 were found to have at least one useable end and 5 were rejected. (We have not experienced the same difficulty with commonly available acrylic blocks, which appear to be fabricated more accurately.)