sensitiveness of Michelsoris 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. 



