308 J. RHEINBERG ON BLACK AND WHITE DOT PHENOMENON. 



All things considered, and notwithstanding that, owing to im- 

 perfect data, it is sometimes difficult to assign the exact cause of 

 the appearances presented by any special case, I think you will 

 admit that there are very substantial grounds for assuming the 



CANADA BALSAM 

 fJL = I.SJl 



WATER 

 fJL ■ L^%6 



STVRAX 



nOMO BROMIDE 

 of HAPKTMAUN 

 ^ « 1.6^0 



A(R 

 /*../ 



REALGAR 

 ^'2S 



"TO^^ 



Fig. 3. 



Variations of Critical Angles and of the Amounts of Light totally reflected, 

 according to the medium in which the diatom is mounted. The 

 Refractive Index (fi) of the diatom silex is taken as 1"43. 



black and white dot phenomenon to be simply due to differences 

 of refractive index of the transparent media. The measure of 

 these differences is the critical angle, or angle of total reflection, 

 and how much this varies for diatoms mounted in the usual 

 media is seen in the accompanying figure (Fig. 3). 



Joxtrn. Qi'.elett Microscopical Club, Sei: 2, Vol. VIII., No. 51, November 1902. 



