48 DISPLACEMENT INTERFEROMETRY BY 



25. Fringes from rough surfaces. Experiments were now made with the 

 use of the air-path compensator (fig. 21,) placed in the d rays when these 

 rays were reversed. A magnificent set of achromatic fringes were here 

 found, only about five in number, with the central members in black and 

 white. Tests similar to the above showed that they are Fresnellian inter- 

 ferences. To prove this the objective of the collimator was removed and 

 even more brilliant fringes were found on placing the washed slit-images in 

 contact. If these patches of light were slid over each other horizontally, 

 by moving the adjustment screw for rotating the micrometer mirror on a 

 vertical axis, the fringes rotated nearly 180, passing from vertical hair-lines, 

 through a maximum of coarseness for the horizontal fringes, back to hair- 

 lines again. On focussing the telescope on the slit it was then found that the 

 large horizontal fringes corresponded to coincident slit-images in focus, 

 whereas for the very fine fringes the focussed slit-images are far apart. No 

 fringes appear on the slit-images in focus, in any case. They lie in front 

 of and behind the image plane. This is exactly the case found above, ex- 

 cept that here the edges of the washed slit-images are exchanged. 



The endeavor to obtain the fringes without the slit was next tried. For 

 this purpose a ground-glass screen illuminated by sunlight (a, fig. 17) was 

 first placed in front of the slit 5 in the absence of the objective (L) of the col- 

 limator. The fringes were still very prominent, though the light was darker. 

 The slit 5 was now also removed. The fringes could then no longer be seen; 

 but on narrowing down the illuminated ground-glass screen a to a vertical 

 strip of light i to 2 mm. broad, they were unquestionably present. In such 

 experiments, therefore, the chief function of the slit 5 is to cut off the light 

 which does not interfere, so that the fringes are lost in the glare. In the 

 absence of such excess of light the fringes are quite visible and therefore 

 certainly always present. By aid of the offset air compensator huge achro- 

 matic fringes may be easily produced ; but they are so sensitive as not to be 

 manageable in an improvised apparatus. 



A number of measurements were now made with the achromatic fringes 

 set at convenient (small) size by the air-compensator. In this work the 

 plate PP' (fig. 23) was moved in three steps over about i cm. For each 

 step a set of data was investigated. The results were in succession 



f N f AA r Xio 3 = o.o 1.9 6.1 12.4 17.8 21. 9 cm. \ 2&N 



(*) 1 A V 1 J- 



(AXio 3 = o.o .4 1.2 2.3 3.3 3. 9 radian] 



, . j A/VXio 3 = o.o 7.1 12.2 19.2 26.6 32.6cm. 1 2A^ 



(2) 1 , . = Q.6 



[ Aa X io j .0.0 i.i 2.2 3.7 5.2 6. 3 radian J & a 



, . f A/V Xio 3 = o.o 9.3 19.2 27.7 37. 7 cm. 1 2AA/" 



[ Aa X io 3 = o.o 1.8 3.7 5.9 7.4radian j a 



The coefficients so obtained are practically identical ; and they agree as nearly 



