166 PROFESSOR KELLAND, ON THE 



Hence, the expression for the vibration is 



cf dy sin — - (vt — B + * '' y J 



In the same way, the vibration due to the second mirror is 



- /*» , sin '- » i4ff ^ sin »T (,< _ g - a + /> </ "A 



ir(p+f) X b X V 2b ) 



B" being equal to Z? + 33 : 



so that the whole vibration at the point M is 



Msin^-Z? ^-^-^ ) 

 XV 2b J 



where M and N are the coefficients of vibration due to each mirror 

 respectively. 



If we expand these in terms of the common circular arc, which is 



