TERMINOLOGY AND CLASSIFICATION OF DIATOMS. 89 



screening oneself from reflected light and giving several quick turns to 

 the fine adjustment screw, the variations in the luminous intensity of 

 the object enable very minute differences of refractive index to be 

 calculated. 



It is indeed easy to calculate a difference of o.ooi without any ambiguity. 

 To obtain a good measurement, it is well to use monochromatic light, the 

 dispersive power of the liquid never being the same as that of the 

 solid. From this it follows that it would always be known, at least 

 to about 0.001 whether a crystal has a greater or less index of refraction 

 than that of any given liquid. This is to a large extent sufficient to 

 diagnose a mineral. 



The following will shew how Mr. Brun's method can be applied 

 to prove that the beads on diatoms are alveoles : 



A large species of Coscinodiscus is chosen and examined in pure 

 water. The beads are then seen to have a less refractive index than 

 that of the silicious skeleton, for on raising the tube of the microscope 

 they are seen to have a dark centre. 



The same species examined in styrax shews that the refractive index 

 of the bead is greater than that of the silicious skeleton, for by raising 

 the tube they are seen to have a brilliant centre. 



This double experiment proves, therefore, that the bead always 

 assumes the refractive index of the medium in which the diatom itself 

 is placed ; now if the bead were solid this could not be the case, and 

 consequently it follows that the bead must be a cavity. Even as far back 

 as 1885, in my "Synopsis," I had arrived at the conclusion that alveoles 

 existed, among other reasons because of the appearance that the valves 

 assumed in the yellow medium. Mr. A. Brun's method is more complete 

 than my own, and both confirms and establishes it. 



Lastly, Mr. Deby, Mr. Cox, Mr. Miiller and myself have endeavoured to 

 elucidate the structure of valves by examining the fragments of broken 

 valves. The study of these fragments which was pursued with objectives of 

 the greatest perfection (among which may be specially mentioned the 

 i-ioth Apochromatic of i*6o N.A. of Zeiss ( J ) ) mounted in the most 

 refractive media, such as monobromide of naphthalin, iodide of methylene 

 saturated with sulphur, the medium 2-4, etc., the injection of coloured liquids 

 as well as the examination of numerous preparations and photographs, 

 for which I am indebted to the kindness of Mr. Haughton Gill and 

 lastly the careful examination of an admirable preparation by Flogel, 



( I ) See Dr. H . Van Heurck. La nouvelle combinaison opticjue de M. Zeiss et la structure de la 

 valve des diatom^es, Anvers, 1890, in 8vo, with plates. 



