28 DISPLACEMENT INTERFEROMETRY. 



It follows, therefore, that the relative thickness of the glass paths traversed 

 by the interfering beams can alone be of influence in shaping the fringe pat- 

 tern in the manner in question. This is in consonance with the general 

 theory of achromatic fringes, the result being a superposition of the color 

 phenomena due to the dispersive refraction of the glass and the colors re- 

 sulting from the wave-lengths of the interfering rays. To test this the ap- 

 paratus, figure 10, is particularly convenient, as the fringes are easily found. 

 Moreover, both rays, a and c, from the collimator at L, eventually pass 

 through the plate N' before reaching the telescope at T. It is thus merely 

 the thickness of the half -silvers M and N, both at 45, that is here in ques- 

 tion. If this thickness is the same, the sharp symmetrical design of but 

 two strong fringes appears. If the difference of thickness is but little over 

 0.5 mm., many fringes, non-symmetric in distribution, are the rule. If the 

 differential thickness is several millimeters there may be hundreds of fringes. 

 If these are small they may be enlarged at pleasure; but they are always 

 faint and useless for measurement. 



