36 DISPLACEMENT INTERFEROMETRY BY 



Since AA r /Aa = - ^ = 23.3 the observed data are above the computed 

 2 cos i 



values, but not more so than the difficulties of these measurements on an 

 improvised apparatus imply. A much more refined method for finding Ac* 

 is, of course, essential. 



19. Interference from rough surfaces. The question now at issue is 

 whether the interferences can be retained when the collimator is removed and 

 the light comes directly from a ground-glass surface or a Nernst filament. The 

 spectrum fringes go at once when the slit is widened ; not so the achromatic 

 sets. Having produced them clearly with sunlight, I found that a ground- 

 glass screen or a scratched mica film could be placed at c or b or a, figure 17, 

 whereas 5 is the slit and L the coliimating lens. 

 The fringes should be transverse, as in figure 16, 

 as vertical fringes are too easily confounded with 

 the white slit-image. The slit was now broadened 

 or quite removed; but the fringes, though less 



prominent from excess of non-interfering light, remained in place distinctly 

 and without other change. On removing the lens, however, the fringes invariably 

 vanished . 



I now replaced the sunlight by the light of a Nernst filament, under the 

 impression that ground glass might to a small degree still behave like plate 

 glass. The same experiments were made, the filaments at e (fig. 17) replacing 

 the ground glass. In this case, however, I first removed the lens L and it 

 was then seen that the two washed slit-images were not superposed, as is 

 otherwise obvious; but it accounts for the failures of the experiments with 

 sunlight. Superposing the two vague images both out of focus, a position 

 was soon found in which the achromatic fringes appeared brilliantly. The 

 slit could now be widened or removed at pleasure, yet the fringes persisted 

 strongly, but with loss of brilliancy. 



It is thus possible to obtain these achromatic fringes directly from the 

 Nernst filament and without a collimator; but they are so mobile, with 

 change of Aa and AJV, that to find them it is necessary first to produce the 

 spectrum fringes with collimator and spectro-telescope ; then to find the 

 achromatic fringes on removing the spectroscope; next to remove the lens 

 of the collimator and adjust for superposed images; and finally to remove 

 the slit. These non-collimated achromatic fringes are best seen in a par- 

 ticular focal plane of the telescope and they change their focal plane with 

 displacement (Aa, A/V). They practically cover the whole width of the 

 washed slit-image. They usually measure about 0.5 in width, but the 

 streamers may extend laterally five times further, depending on the adjust- 

 ment. When pronounced, the slit -images may even be separated as in figure 

 1 8, while each alone retains the achromatic fringes. This puzzling phenom- 

 enon, which I had previously obtained, is probably due to the intersection and 

 interference of rays in a region in advance of the plane of vision. Finally, as 



