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XXVIII. Interference Phenomena in a new Form of Ref Tacto- 

 meter. By Albert A. Michelson*. 



IN an experiment undertaken with a view to detecting the 

 relative motion of the earth and the luminiferous aether 

 ('American Journal of Science/ No. 128, vol. xxii.), it was 

 necessary to produce interference of two pencils of light which 

 had traversed paths at right angles with each other. This was 

 accomplished as follows: — The light from a lamp at a (fig. 1) 

 Fi<r. 1. 



was separated into two pencils at right angles, b c and b d, by 

 the plane-parallel glass b; and these two pencils were returned 

 to b by the mirrors c and d, whence they coincide along be, 

 where they are viewed by a small telescope at e. It is evi- 

 dent that, so far as the interference is concerned, the apparatus 

 may be replaced by a film of air whose thickness is bc—bd, 

 and whose angle is that formed by the image of d in b, with c. 



The problem of interference in thin films has been studied 

 by Feussner; but his equations do not appear to give the ex- 

 planation of the phenomena observed. In particular, in the 

 Annalen der Phydk und Chemie, No. 12, 1881, on page 558, 

 the conclusion drawn from the preceding equations is that the 

 interference-fringes are straight lines; whereas in the above 

 apparatus they are in general curves, and there is but one case 

 — that of the central fringe in white light — which is straight. 



I have therefore thought it worth while to attempt the solu- 

 tion of the problem for a film of air for small angles of inci- 

 dence, and neglecting successive reflections (which in the 

 * Communicated by the Author. 



