50 DISPLACEMENT INTERFEROMETRY BY 



fringes and are seen with the slit-image out of focus are also difficult to 

 find because of the short length of the slit-image. As first obtained they 

 lacked brilliancy and were not easily observed. Similarly, experiments 

 with filaments failed to show the fringes, although made in parallel with the 

 successful result with sunlight. 



A considerable assistance in finding the fringes is an opaque screen with 

 a vertical slit 2 mm. to 4 mm. wide placed just in front of the objective of 

 the spectro-telescope, in the best position as to symmetry. This screen cuts 

 out rays which do not interfere and makes the fringes stronger, even though the 

 background is darker. Fringes are frequently found in this way when they 

 are all but invisible in the full spectrum. 



In addition to the regular fringes, a much larger vague set seems to be 

 present in another focal plane. They also rotate, etc., like the regular 

 fringes, but the experiment led to no decision with regard to them, as 

 they appeared in the field erratically and could not be produced at will. 

 They may be shadow interferences of the principal set. 



An attempt was made to register slight lateral displacements of the slit 

 in terms of the displacement of fringes, but as the slit-images are thrown out 

 of coincidence when the slit moves, trustworthy numerical data can not be 

 obtained. One may estimate that as a first approximation 



d ^ 

 x = \ 



c 



if x is the lateral displacement of the slit and c the distance between mirrors 

 m and n. Hence 



400 



x = - 6Xio~ 5 = o.oo24 cm. 

 10 



should have been equivalent to the passage of one fringe in the given appa- 

 ratus, or generally 



2AAT cos (/3-f 7)/2 =cx/d 



If (j3+7)/2 = 70, A7V=io~ 4 cm., and d/c = 4o, 

 then 



. = 2.Xio~ 3 cm. 



could have been registered. Incidentally it appears that two vertical lines 

 of the slit, xo/2 =0.0014 cm. apart, would wipe out each other's interferences; 

 but this is not the case, as much greater slit-widths are admissible. To the 

 right and left of the line of the slit capable of producing interferences the 

 parallel lines either cease to produce parallel rays, or parallel rays come 

 from symmetrical but different lines. 



After completing these experiments, the distance between slit S and the 

 mirrors m and was increased to about 9 meters. The same lens doublet, 

 focussing a large solar image on the slit, was used as before. With the aid of 

 the slotted screen in front of the telescope and the micrometer distances 

 from the preceding experiment, the fringes were found without difficulty. 



