100 C. Barus — Interferometry of Air. 



ascertain whether the self-compensating case or the other is 

 the one which succeeds. In fact the four lines m in' n' n 

 are not necessarily coplauar, but will lie on a ruled surface, 

 which is to be made as nearly plane as possible. Thus the 

 plates P and G may be clamped to a long strip of plate glass, 

 or other devices employed. These succeed adequately after 

 some readjustment, but I have not been able to place the cen- 

 ter quite in the center of the field of view. Experimentally 

 this is not necessary ; for the eye is quite as sensitive in placing 

 the horizontal element of the circle in coincidence with the 

 fiducial sodium lines (which it intersects at right angles), the 

 inclination on the two sides being in opposite directions. ..Nat- 

 urally the interference bands should be strong. 



The micrometer screw of M, in this case, bisects the acute 

 angle of the rhombus of <jf> = 30°, and the motion of the mirror 

 over A N here cuts off 2JtV/cos15° from the beam of light. 

 Moreover, at the grating the beam of light is shifted laterally 

 when M advances, by an amount AN tan 15° so that 2JN sec 

 15° sin 2 15° is restored. Thus the path difference produced 

 is generally 2Jy = 2AJV cos <£/2. 



5. The Same. Results. — The data obtained in case of the 

 rhombus are given in Table II. 



Table II. 



Glass tube as in Table I. Barometer, 76 - centimeters. 



p o p 10 5 AiV 10 5 Ay (i-l 



76-0 -4 1285 1241 287 



1280 286 



The data are of the same order as the preceding table, remem- 

 bering that the tube is not twice traversed by the beam of light, 

 as is the case with the adjustment of Tab|e I. 



The current from the induction coil was now passed through 

 the exhausted tube and the circuit made and broken. Not the 

 slightest effect could be observed, the interferences remaining 

 stationary in all parts of the field, just as in the preceding case. 



The endeavor must now be. made to exhaust the tube to the 

 highest degree possible, and to pass the current between elec- 

 trodes of very high. potential difference. To this I hope to 

 return at some other opportunity. 



Brown University, Providence, R. I. 



