12 



DISPLACEMENT INTERFEROMETRY. 



Hence 

 whence 



io 3 AA T =5.o2 cm., or 10^ = 3.91 cm. 



= 149 cm. 



0.00781 



The fringes in the former case were not large enough to admit of better 

 agreement. 



These experiments were made merely for the purpose of showing that there 

 is no difficulty, so far as the fringes are concerned, in the successive substitu- 

 tion of plates and lenses. The pin e should be removed from contact by an 

 auxiliary lever during this exchange and the apparatus gently tapped before 

 a reading is taken. 



The lens tested by the ocular micrometer (here io 3 AJV = o.o37Ae) showed at 

 the left center and right side readings of i.o, i.o, 0.8 scale-parts, respectively. 

 Thus the three values of io 6 Ax are 28.9, 28.9, 28.4 cm., respectively. The dif- 

 ference is but 5 X io~ 6 cm., the positions taken being about a centimeter apart, 



A concave lens was next tested, giving the readings 



Lens 0.0277 0.02895 cm. 



Plate .0216 .02280 cm. 



AN .0061 .00615 cm. 



Thus A# = o.oo478 cm., r=i.o8 cm., as before, and therefore 



R= 122 cm. 



The front and reversed sides of the lens calipered did not differ by more than 

 A% = 3Xio~ 6 cm. 



In the case of two magnifying lenses the readings obtained on different 

 days were (all data in centimeters) . 



In case of such large displacement (nearly a centimeter) the fringes are 

 liable to rotate considerably unless they are vertical. The latter should there- 

 fore be selected ; otherwise the allowance for rotation may be troublesome. 



Though the slides were quite inadequate for precise measurements of this 

 kind, I was nevertheless able to test the method of figure 4 and equations (8). 

 Three readings ^ = 0.325 cm. apart gave, respectively 



^ = 0.0148 y = 0.01975 y = 0.0260 cm. 



Hence 



2AAf = 2y' (y-\-y"} = 0.0013 cm. 2Ajc = o.ooioi cm. 



Thus R=io6 cm. The observations, but for the defective slides, were very 

 satisfactory. 



