On Transmitted Illumination. 93 



ON TRANSMITTED ILLUMINATION IN CONNECTION 



WITH THE STRUCTURE OF THE SILICEOUS 



DIATOM VALVE. 



BY EICHAED BECK. 



It is undoubtedly a remarkable fact that, after all the care- 

 ful microscopic examination which has been bestowed for so 

 many years upon the siliceous valves of the Diatomacese, no 

 person has yet been able to give any explanation of their 

 structure, which could universally be received as satisfactory. 



A few observers appear to have had no difficulty in satisfy- 

 ing themselves as to the cause of those markings which are so 

 familiar to every microscopist ; but the opinion of almost every 

 one, who has refrained from any published statement, is as 

 undecided as possible. 



I believe the conflicting ideas of some, and the uncertain 

 opinions of others, are easily accounted for by the peculiarities 

 of the case ; of these I would especially draw attention to that 

 of the siliceous valve being formed of a perfectly transparent 

 material, and consequently when light is transmitted through 

 it, the markings appear because of the refraction or the reflexion 

 of the light by the various irregularities of structure, and not 

 because of the dispersion or absorption of the light, which so 

 invariably and materially assist us in the determination of 

 almost all natural objects. 



Again, it is very seldom indeed that any truly transparent 

 natural object is presented to the naked eye in the ordinary 

 course of our existence, and still less, frequently, under such 

 circumstances as to have a direct light transmitted through it ; 

 the only instances that occur to me just now, are the drops of 

 falling rain, or flakes of snow as seen against a light sky, and 

 consequently the eye from not being accustomed to the appear- 

 ances of transparent objects, is quite unable to come to any 

 judgment upon their appearances. 



It has frequently occurred to me, and probably to many 

 others, that some objects of a tangible size might be found to 

 show the difficulty of determining the structure of transparent 

 objects from the appearances produced by passing light through 

 them, and it was quite unexpectedly that I met with a common 

 small glass tumbler, the lower portion of which was covered 

 with small hemispheres. The difficulty of determining the 

 structure even of this large object, when looking perpendicularly 

 though the surface, is at once apparent, and may be seen as 

 well in the accompanying engraving, copied from a photograph, 



