262 J. W. Gibbs—Double Refraction and the 
ArT. XX VII.—WNotes on the Electromagnetic Theory of Light ; 
by J. Wittarp Grpps, Professor of Mathematical Physics 
in Yale College. Double Refraction and the Dis- 
Onis 
persion of Colors in perfectly transparent Media. 
1. In calculating the velocity of a system of plane waves of 
homogeneous light, regarded as oscillating electrical fluxes, in 
transparent and sensibly homogeneous bodies, whether singly 
or donbly refracting, we may assume that such a body is a 
very fine-grained structure, so that it can be divided into parts 
having their dimensions very small in comparison with the 
wave-length, each of which may be regarded as entirely similar 
to every other, while in the interior of each there are wide dif- 
light of any one color. 
In this investigation, it is assumed that the electrical displace- 
wave-length and the period is the same for stationary as for 
progressive waves is’ evident from the consideration that a syS- 
