488 



Transactions. — Gheviistry and Physics. 



Let the thickness of the slab be t, and let a small element 

 of the wave-front be AB (fig. 1), measured parallel to the 



Rare 



03 

 < 



CO 



Den 



se 



Fig. 1. 



plate, but at right angles to the initial plane. Let this wave 

 emerge from the plate at CD, and let its front be now DM, 

 making an angle 8 with the face of the plate. I^et the refrac- 

 tive index along CD he. jx, and that along AB be fj}. The plate 

 is supposed immersed in air of refractive index unity. We 

 may consider the element DM to be plane. 



Let AB = X ; 

 Then time taken to travel path BD = k^-t ; 



Tmie taken to travel path AM = kfj}t -\- A;CM. 

 Whence BM = /x^ - ^jlH. 



Sin 8 = 



BM 

 BD 



fit — fj}t 



if we neglect internal refractions. 



From this it follows tbat, in order that the wave may 



remain plane, we must have /*■ = constant (= A). 



ax 



Then the deviation 8 is given by 



Sin 8 = \t. 



Or, if 8 is small, 8 varies as t (1.) 



2. Experiment. (Fig. 2.) 

 In order to test this experimentally the prisms were 

 removed from a spectrometer, whicli was tlien fixed with its 



