THE 



AMERICAN JOURNAL OF SCIENCE 



[THIRD SERIES.] 



Art. XXIII. — On the Feasibility of Establishing a Light-wave 

 as the Ultimate Standard of Length; by Albert A. 

 Michelsojst and Edw. W. Morley. 



In the problem of making a light wave a practical standard 

 of length, it is desirable to use as an intermediate standard a 

 metal bar bearing two plane surfaces at the greatest distance 

 apart at which interference (between two pencils having this 

 difference of path) is accurately measurable. Some preliminary 

 experiments have shown that in the case of the green mercury 

 wave the distance may be a fourth of a meter. To keep well 

 within limits, however, it is safe to say that an intermediate 

 standard one-eighth of a meter long is entirely practicable. 

 Such a distance will contain about a quarter of a million waves. 

 The fractions of a wave present no especial difficulty ; but to 

 find the whole number when it is so large is not so simple a 

 matter. 



The following plan has been adopted and it is believed that 

 by its means the chances of error are reduced to a minimum. 



A series of intermediate measures were constructed alike in 

 all respects save that the distances between the two planes were 

 made very nearly 2 -11 , 2 -10 , 2 -9 ; 2 -8 , 2 -7 of a meter. 



A description of one of these will answer for all. It consists 

 of a brass bar provided with two sets of brass studs against 

 which are pressed by spiral springs two silvered glass pieces 

 two cm. square and about 0*6 cm. thick. The reflection takes 

 place from the front surface, and the distance between these 

 surfaces was adjusted as follows : — 



Am. Jour. Sci — Third Series, Vol. XXXYIII, No. 225.— Sept., 1889. 

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