52 



DISPLACEMENT INTERFEROMETRY. 



is too feeble for detecting fringes so elusive as the present. The achromatics, 

 however, are strong and brilliant even here (Nernst filament) . 



An interesting result is obtained in case of the achromatic fringes by nar- 

 rowing one of the beams, for instance that coming from the mirror m (fig. 

 26), by a screen with a vertical slit about 2 mm. wide. 

 In such a case the slit-image (out of focus) is correspond- 

 ingly narrowed. It may be passed from side to side of 

 the broad washed slit-image coming from the mirror n, 

 by moving its adjustment screws (vertical axis). The 

 fringes then appear only in a particular position of the 

 narrow image in the field of the broader ; but when they 

 do appear they spread far beyond the margins of the nar- 

 row image on both sides. Interference thus apparently 

 occurs where but one beam is present. The phenom- 

 enon is like those instances above (figs. 18, 24, Chapter 

 II) and means, as I understand it, that the beams have 

 met in some other focal plane, though one is tempted to 

 conclude that interference is stimulated by resonance, 

 in particular as it is often impossible to find a plane in 

 which they have met. The achromatics may sometimes be seen before and 

 behind the principal focal plane, but more frequently either in the one or in 

 the other region only. 



71' 





