sensitiveness of Michelsons Interferometer. 
379 
adjusted so as to give an even shade, and half of one mirror 
should be covered with a thin plate which causes a small retarda- 
tion (or a plate may be cut in two and one half slightly tilted). 
If now the whole of the light be retarded by a further accurately 
measurable amount (say by inclining a moderately thin plate 
placed in the path of the light and provided with circles or with 
mirror and scale) the two halves of the field will become dark in 
close succession, and all the above-mentioned conditions of sensi- 
tiveness are satisfied. There is still the difficulty of the unequal 
intensities of the interfering beams, and the difficulty can be 
overcome as before, without the counterbalancing difficulty due to 
change of phase being met with, for we may here use plane 
polarised light. 
