IN a former publication 1, an account was given of the examination of the topographical structure of a cleavage face of a selenite crystal, use being made of multiple-beam Fizeau fringes. Only low ...

IF a pair of optically flat plates, making a wedge angle of a few seconds, be placed in the beam from a collimator with a cross slit (to obtain effectively a point source) and the eye be placed at the ...