THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL 



Vol. XIII. 



OCTOBER, 1892. 



No. 10. 



CONTENTS. 



Index of Refraction (illus- 

 trated). — AuBERT - . . - 225 



Diatoms of the Connecticut 

 Shore. — II. — Terry . - - - 230 



The Protozoa — a Phvlum of the 

 Animal Kingdom Considered 

 Biologically (illustrated). — 



OSBORN -------- 233 



A Bacteriological Potato-Sec- 



tion Cutter (illustrated). — 

 Dawson - - - . . . . 243 



Bacteriology. — What Has 

 Been Done in Bacteriology in 

 Relation to Hygiene - . - 245 



Microscopical Societies. — 

 St. Louis. Lincoln . - - - 247 



New Publications. — Bausch & 

 Lomb. Stockwell - - - - 248 



Index of Refraction. 



By a. B. AUBERT, M.S., 



ORONO, ME. 



The usual detinition of index of refraction is somewhat as 

 follows : the index of refraction of a substance is equal to the 

 quotient of the sine of the angle of incidence divided by the sine 

 of the angle of refraction. 



A simple figure will readily make this clear. 



Let A B CD (figure i) represent the outline of a circular 

 vessel, A C being the water-line. When the beam of light is 

 incident along B E, which is perpendicular to A C, thei^e is no 

 refraction; that is, the beam passes from one medium (air) into 

 the other (water) without being bent or refracted. When it is in- 

 cident along m E, there is refraction, it is bent at E toward the 

 perpendicular (or normal) B D, and strikes the circle at n. When 

 it is incident along m E there is also refraction at E, the beam 

 striking the circle at n' . From the ends of the incident beams, 

 let the perpendiculars tn o, m o be drawn upon B D, and from 

 the ends of the refracted beams let the perpendicular p n and p' n 

 be also drawn. 



