28 THE INTERFEROMETRY OF 



to overlap considerably (by fore-and-aft motion of P) the fringes become 

 fine and vertical and the parallel blades of light, which interfere at the focus 

 of the telescope, are 0.5 to i cm. apart at the objective. 



In further experiments, screens 5, s' (fig. 14) were placed in the paths of 

 the pencils b b f , so that they were compelled to pass through vertical slits 

 0.5 mm. wide in the screens. In this way the interfering rays were identified. 

 The first vertical hair-line fringes came from rays about 5 mm. behind the 

 edge of the prism P'. Hence the pencils were here about 1.2 cm. apart when 

 they entered the telescope. The largest and last of the fringes came from 

 close to the edge of P'. The experiment was varied as follows: Supposing 

 both screens s and 5' placed as far to the rear as the visibility of fringes per- 

 mits; let the former, s, be slowly pushed forward. The fringes then contract 

 from the very broad set, figure 16, case i, to the strong and narrow set 2 

 (which is a mere line for a full wave-front), and then expand again to case 3. 

 If, now, 5 is left in place and s' moved forward slowly in the same way, the 

 identical contraction and expansion, cases i, 2, 3, are reproduced. The screen 

 s' may then be left in place and 5 in turn slowly moved forward with the same 

 results, etc. (there may be 6 alternations), until finally the effective parts of 

 the pencils b and b' are beyond the edge of the prism P'. In case 2 the two 

 slits 5 and s' are obviously symmetrical to the interfering rays, whereas in 

 cases i and 3 the diagonally opposite edges of the slits s and 5' limit the effi- 

 cient pencils to a sheet. If the edge of the prism were truly a knife-edge, the 

 last fringes would be very large, since the distance cc', figure 14, would vanish. 

 If the fringes are vertical (obtained by tilting P around an axis parallel to 

 LT), the case 2 is given by 2 or 3 strong vertical lines, whereas i and 3 

 consist of 10 or 20 lines, all of about the same width and distance apart. If 

 the slits s, s' are finer (i mm.), the fringes are throughout sharper. A single 

 displacement, i, 2, 3, corresponds to about 2 mm. When the edge of P' is 

 approached the case 2 often shows vertical strands of fringes, a strong central 

 strand, and two or three fainter ones either side of it. The cases i and 3 are 

 not stranded. 



A similar result (passage of case 2 into 3, fig. 16) may be produced by 

 moving P forward, the case 3 appearing just before the pencils b b' leave the 

 edge of P'. Again, when M is moved rearward, when both b and b' are near 

 the edge of P', the cases 2 and 3 are obtained. In general, the width of the dif- 

 fraction pattern increases without changing the size of fringes, as the width 

 of the available wave-front decreases. A similar result will be described in 

 connection with figure 48, Chapter II. Naturally, if the displacement is 

 considerable, it is accompanied by some rotation of fringes. 



15. Displacement parallel to rays. It now becomes of importance to test 

 the range of displacement as modified by the angle of reflection, increasing 

 from 5 = o. It is therefore desirable to make a few direct measurements. The 

 angle 6 at P, figure 14, was found to be about 49 45', so that the total angle 

 at M is d = 40 1 5'. M and N are both on micrometers, with the screws normal 



