with tlie Aid of a Grating. 



415 



as a whole to tlie right or to the left, while the rings partake 

 of the customary motion toward or from a centre. The 

 horizontal motion in such a case is of the nature of a coarse 

 adjustment as compared with the radial motion, a state of 

 things which is often advantageous. The large divisions of 

 the scale are not lost, in other words. Moreover, the dis- 

 placements may be used independently. 



The two motions are coordinated inasmuch as violet travels 

 toward the centre taster in a horizontal direction, i. e. at a 

 greater angular rate, than red. Hence the ellipses drift 

 horizontally but not vertically. Naturally in the two 

 positions specified above for ellipses, the fringes travel in 

 opposite directions for the same motion of the micrometer- 

 screw. As the thickness of the grating is less the ellipses 

 will tend to open into vertical curved lines, while their 

 displacement is correspondingly increased. With the grating 

 on a plate of glass about e = '6S cm. thick, and having a 

 grating space of about D = '0i)03f>l cm., at an angle of 

 incidence of about 45°, the displacement of the centre of 

 ellipses from the D to the E line of the spectrum corresponded 

 to a displacement of the grating parallel to itself of about 

 *006 cm. It makes no difference whether the grating side 

 or the plane side of the plate is toward the light or which 

 side of the grating is made the top. If the grating in 

 question is stationary and the mirror N alone moves parallel 

 to itself along the micrometer-screw, a displacement of 

 N = '01 cm. rouohly moves the centre of ellipses from D to E, 

 as before. This displacement varies primarily with the 

 thickness of the grating and its refraction. It does not 

 depend on the grating constant. Thus the following data 

 were obtained with film gratings (on different thicknesses e 

 of glass and different grating spaces D) for the displace- 

 ments, N, of the mirror at N, to move the ellipses from the 

 D to the E line, as specified : — 



Glass grating, ruled 



e ='68 cm. 

 e =-57 ,, 

 e=-24 „ 

 e=-48 „ 

 e'=-24 „ 



X/D = 



•168 

 •352 

 •352 



•352 



N=010em. 

 •008 „ 

 •003 „ 



•003 „ 



Film on glass plate 



Film on glass plate 



Film between glass plates J 



Reduced linearly to e='6S cm., the latter data would be 

 N / = , 010 and N'^'009, which are of the same order and as 

 close as the diffuse interference patterns of film gratings 

 permit. The large difference in dispersion, together with 



