380 A. A. MicheUon and E. W. Marley — Influence of 



This, together with the fundamental importance of the work 

 must be our excuse for its repetition. It may be mentioned 

 that we have tried to obtain formulated objections to these 

 experiments but without success. The following are the only 

 points which have occurred to us as being susceptible of im- 

 provement. 



1st. The elimination of accidental displacement of the fringes 

 by deformation of the glass ends of the tubes, or unsymmetri- 

 cal variations of density of the liquid, etc., depends on the 

 assumption that the two pencils have traveled over identical 

 (not merely equivalent) paths. That this is not the case was 

 proven by experiment ; for when a piece of plate glass was 

 placed in front of one of the pencils and slightly inclined, the 

 fringes were displaced. 



2d. The arrangement for producing the motion of the medium 

 necessitated very rapid observation — for the maximum velocity 

 lasted but an instant. 



3d. The tubes being of necessity of small diameter and only 

 their central portion being available (since the velocity dimin- 

 ishes rapidly toward the walls) involved considerable loss of 

 light — which, having to pass through a slit was already faint. 



4th. The maximum velocity (in the center of the tube) 

 should be found in terms of the mean velocity. (Fizeau con- 

 fessedly but guesses at this ratio.) 



These are the suggestions which determined the form of 

 apparatus adopted, a description of which follows : 



The Ref Tactometer. — After a number of trials, the following 

 form was devised and proved very satisfactory. Light from "a 

 source at a (fig. 5) falls on a half silvered surface b, where it 

 divides; one part following the path bcdefbg and the other 

 the path bfe clcbg. This arrangement has the following advan- 

 tages : 1st, it permits the use of an extended source of light, as 

 a gas flame; 2d, it allows any distance between the tubes 

 which may be desired ; 3d, it was tried by a preliminary ex- 

 periment, by placing an inclined plate of glass at h. The only 

 effect was either to alter the width of the fringes, or to alter 

 their inclination ; but in no case was the center of the central ichite 

 fringe affected. Even holding a lighted match in the path had 

 no effect on this point. 



The tubes containing the fluid were of brass, 28 mm internal 

 diameter ; and, in the first series of experiments, a little over 

 3 meters in •length, and in the second series, a little more than 

 6 meters. The ends of these tubes were closed with plane 

 parallel plates of glass which were not exactly at right angles 

 but slightly inclined so as to reflect the light below the teles- 

 cope, which would otherwise be superposed on that which 

 passed through the tubes. The tubes were mounted on a 



