1882.1 



MICKOSCOPICAL JOUKNAL. 



45 



addition I have at times noticed a 

 peculiar clear space adjoining one 

 side of a diatom, as if a drop of some 

 invisible oil was attached to it, while 

 everywhere else the dirt and loose 

 matter in the water adhered to the 

 frustules. The same clear space is 

 often seen around other particles, such 

 as sand and dead diatoms, etc. If a 

 layer of jelly-like substance did in 

 fact surround the living diatom, and 

 had the power of wave-like motion in 

 its substance, it might give rise to 

 motion of the diatom, like the creep- 

 ing of a snake; but it seems impossi- 

 ble that a layer of such substance suf- 

 ficient to accomplish the result, es- 

 pecially in large diatoms, should es- 

 cape detection, and the alternation of 

 motion would still be unexplained. 



Fig. 13. 



But in some cases, at least, the shape 

 of the diatom has an influence upon 

 its motion. Amphiprora ornata, which 

 has a half twist, moves through the wa- 

 ter endwise, with a rocking motion 

 exactly like that of a vessel in the 

 trough of the sea, and the curved 

 webs or projections on the sides of 

 the valves would give just such a mo- 

 tion to the frustule when forced 

 through the water, but if prehensile 

 filaments caused the motion the effect 

 of the shape of the diatom would be 

 little or nothing, and in clear water 

 prehensile filaments would affect no- 

 thing unless they could fasten to the 

 shifting atoms of water as to a, solid 

 body. 



In conclusion, I append a diagram 



of the course followed by a very ac- 

 tive frustule of Cymbella cuspidata ob- 

 served moving in clear water: the 

 diatom is drawn at the starting point, 

 and the arrows indicate the course in 

 which it moved. None of the ac- 

 credited causes of motion in diatoms 

 seem to me to explain satisfactorily 

 all the phenomena observed. 



o 



Binocular Microscopes. 



The question is often asked by pur- 

 chasers of microscopes, whether there 

 is any real advantage in binocular 

 over monocular instruments. In so 

 far as we regard the question in its 

 purely practical aspects, it is capable 

 of a definite and a satisfactory an- 

 swer; but when we come to decide 

 upon it from theoretical 

 grounds — to state just what 

 effect any particular binocu- 

 lar arrangement will have 

 when applied to the examina- 

 tion of a specified object, to 

 explain how much of the ap- 

 pearance of relief is real, 

 and how much is merely a 

 mental impression produced 

 by the two images in the two 

 eyes — the problem presented 

 is a very difficult one. 



We shall, therefore, con- 

 fine ourselves to the practical 

 side of the subject in this contribu- 

 tion. If the question is, whether 

 there is any advantage in a binocu- 

 lar microscope in studying the 

 form of objects — whether the ap- 

 pearance of relief that it gives is ne- 

 cessary to enable us to form a correct 

 idea of the true shape of objects in 

 which the appearance of relief is most 

 striking — the answer must be a decid- 

 ed negative. It is true that the binocu- 

 lar does reveal more of the form of an 

 object at the first glance, than the mo- 

 nocular; but it is a matter of experi- 

 ence that those who use only one eye 

 in microscopical work, never make the 

 mistake of supposing that an object is 

 flat merely because it seems to be so. 

 A very short experience enables one 



