with the Aid of a Grating. 421 



If e — e\ i. e. for an infinitely thin plate e = Q, 



n\ d = 2y cos a. (8) 



Again, for normal rays y m and y n , cc = and S i — i =/?i- 

 Hence, for a parallel compensator of thickness e', 



n \ =—2y + 2(e- e')nJ cos S x 



0>) 



If yx e = l = /i fl , equation (6) reduces to 



n\ = 2cos a(y + ef cos®), 



clearly identical with the case e = for a different y. 



In view of the complicated nature of equation (6) I have 

 computed path differences for a typical case of light crown 

 glass f. as shown in Table I., for the Fraunhofer lines B, D, 

 E, b\ Gc, supposing the E ray to be normal (a = 0). The 



Table I. 

 Table for Path Difference, — 2(y m — yj cos u + eZ Q + eZ 01 

 = — 2// cos u + eZ. Light crown glass. E rays normal 

 to mirrors. Grating space D = '000351 cm. 1 = 45°. 

 © = 33° 51'. 



Spectrum lines... 



XX10 6 = 



B. 



68-7 



D. 



E. 



F. 



G-. 







58-93 



52-70 



48 61 



43 08 



cm. 





**• = 



1-5118 



1-5153 



1-5186 



1-5214 



1-5267 







V = 



30° 46' 



32° 38' 



33° 51' 



34° 40' 



35° 46' 







°1 = 



19° 47' 



20° 51' 



21° 31' 



21° 57' 



22° 30' 







Oi = 



-3° 5' 



-1° 13' 



0°0' 



0° 49' 



1° 55' 







l~ e l = 



-1° 44' 



-0° 40' 



0°0' 



0°28' 



0°59' 







Pi = 



23° 25' 



22° 17' 



21° 30' 



21° 13' 



20° 49' 







0i-i8i = 



-3° 38' 



-1° 26' 



0°0' 



0°44' 



1°41' 







2 cos « == 



1-9970 



1-9996 



2 0000 



1-9998 



1-9996 



cm. 





Z B = 



•0383 



•0176 



•oooo 



-•0095 



-•021 9 



cm. 





Z e = 



3-2100 



32104 



3-2648 



3-2805 



3-3038 



cm. 



Path Difference* 



},X={ 



-1-9970 



-1-9996 



-2-0000 



-1-9998 



-1-9996 



lorn. 



y=] cm.; 

 e= 1 cm. 



+3-2483 



+3-2580 



+3-2648 



+ 3-2710 



4-3-2819 



The same for 



2// = 3-2(>48 cm. ; 



e = 'Q8 cm. 





-•0079 

 -•0107 



-•0042 

 -•0056 



+-o 



+ 



+ •0040 

 + •0049 



+ 0120 

 + •0156 



cm. 

 cm. 



* The purpose of these data is merely to elucidate the equations. AN refers 

 to the displacement of either opaque mirror, M or N. 



t Taken from Kohlrausch's ' Practical Physics/ Uth edition, 1910, 

 p. 712. 



