In three of the four [negative] experiments, the interferometers have been enclosed in heavy, sealed metal housings and also have been located in basement rooms in the interior of heavy buildings and below the level of the ground; in the experiment of Piccard and Stahel …, a metal vacuum chamber was used … If the question of an entrained ether is involved in the investigation, it would seem that such massive and opque shielding is not justifiable. The experiment is designed to detect a very minute effect on the velocity olight, to be impressed upon the light through the ether itself, and it would seem to e essential that there should be the least possible obstruction between the free ether and the light path in the interferometer …