118 J. RHEINBERG ON THE BLACK AND WHITE DOT PHENOMENON. 



position of two wave-streams in the same direction the white 

 dots become blighter than the diatom substance. I have spoken 

 of trellis -work as if the light issued in parallel beams of equal 

 intensity throughout, whereas actually cones of light of diflferent 

 intensities in different directions are formed. I have scarcely 

 touched on the differences which ensue according to whether 

 the diatom is imbedded in a medium of higher or less refractive 

 index than itself, and what differences the spacing of the per- 

 forations, their thickness and their diameter, causes. All these 

 things can be worked out easily enough, and I am hopeful that 

 some interesting results as to the best mounting media w^ill 

 be obtained by pursuing the subject further. 



Journ. Qutkett Microscopical Club, Ser. 2, Vol. VIII No. 49 November 1901. 



