HARDWICKE'S SCIENCE-GOSSIP. 



though I venture to consider my " Key " of more help 

 in tracking down species than the table. Here is my 



method of arranging the information given in the 

 table under the heading " Actinophryina." 



I. Shell absent 



II. Shell present, /r^t'. 



Actinophryina, p. 14. 



1. Pseudopodia arising from all parts of the surface. Ex. Actinophrys,^. 14. 



2. Pseudopodia arising from a zone near the circumference. Ex. Tricho- 



discits, p. 7S0. 



3. Pseudopodia arising from one side. Ex. Plagiophrys, p. 604. 



1. Incrusted with foreign mattter. Ex. Pleurophiys, p. 606. 



2. Not incrusted, oblong. I f Orihce lateral. Ex Trmema ^. 783. 



' ^ y 0. Unnce termmal. Ex. Euglypka, p. 306. 



III. Shell i:iresent, attached to foreign bodies, Ex. Urmila, p. 795. 



{The references are to the pages of the last edition of the Alicrographic.) 



I have drawn up several of these " Keys," notably 

 one for the Rotataria, which has proved serviceable ; 

 it extends over five full sheets of foolscap written 

 brief-wise. Portions of these tables are merely re- 

 arrangements, similar to the key to the Actiiio- 

 phryitia given above. They suggest that complete 

 sets of "Keys," with references to the pages of 

 the text, printed as an appendix to the future 

 editions of the dictionary would be as useful as the 

 analyses prefixed to Lewis's Law of Trusts, and 

 Smith's Real and Personal Property, the former 

 of which also has references to the pages of the ' 

 text. 



Readers who have followed me thus far will under- 

 stand that I advocate the exercise of drawing up 

 these conspectuses for two reasons : — (i) the practice 

 impresses characteristic features in the mind ; and 

 (2) the conspectus when framed is a handy and 

 practical guide to the worker. Finally I believe the 

 " Micrographic " can be most advantageously em- 

 ployed if the student will take it up, pen in hand, in 

 the way indicated above. 



GOSSIP ON CURRENT TOPICS. 

 By W. Mattieu Williams, F.R.A.S., F.C.S. 



LIQUID PRISMS.— The spectroscope has now 

 become so important as an instrument for 

 research, that everything concerning it is interesting. 

 Its efficacy depends upon the varying refrangibility 

 of light from different sources, or light of different 

 colours. We pass compound light such as that of a 

 white sunbeam through a prism of glass or other 

 transparent substance, and thereby bend it out of its 

 original course. If it were all bent in the same degree, 

 we should still obtain a white image on a screen or 

 in the eye ; but as the violet, indigo, blue, green, 

 yellow, orange and red rays, are refracted or bent in 

 different degrees — in the order named — they diverge 

 from each other in the newly-acquired or bent course, 

 and thus are projected upon different places, and are 

 accordingly seen separated from each other. 



But the bending or refracting power of transparent 

 substances varies considerably, and the degree of 

 dispersion they thus effect varies accordingly. The 

 more delicate researches in spectroscopy demand high 



degrees of dispersion, far more than a single prism of 

 glass can supply. Therefore, trains of such prisms 

 are used, the second receiving from the first, the third 

 from the second, and so on, in such wise that each 

 spreads further out the outspreading of the preceding 

 one. But each of these prisms stops some of the 

 light, and a long battery of prisms is complicated and 

 costly. Therefore it is desirable to use the trans- 

 parent substance which has the greatest dispersive 

 power. When some fortunate chemist shall succeed 

 in crystallizing carbon, or, better still, in obtaining it 

 in the form of a plastic transparent jelly, like many 

 of its compounds, we shall have prisms vastly 

 superior to those of glass, and microscope lenses of 

 corresponding superiority. In the meantime, a liquid 

 compound of carbon with sulphur, the bisulphide of 

 carbon, has been used by enclosing it in a hollow glass 

 prism. This, however, stands a long way behind the 

 diamond in dispersive power, thcjugh somewhat before 

 glass, and it has a pernicious habit of becoming yellow 

 as it grows old, and has done much work by exposure 

 to light. Mr. H. G. Madan has tried, as a substitute, 

 a liquid with rather higher refractive power than the 

 bisulphide, which is also a carbon compound contain- 

 ing nitrogen and hydrogen in addition to the carbon 

 and sulphur, and bearing one of those hideous hypo- 

 thetically descriptive names that are now in fashion 

 among a certain class of chemists, " pkenylthiocar- 

 batnide " and " iso-so/focianatofenilico.''^ It is interest- 

 ing, however, in spite of such ill-usage. It retains its 

 colour ; is far less volatile than the bisulphide ; is not 

 so combustible, and does not dissolve the cement 

 that may be most conveniently used for uniting the 

 three pieces of glass that constitute its prismatic prison 

 walls. Therefore it will come into frequent use. 



Scavengers on the Sea-Coast.— " In the Revue 

 Biologique du Nord de la France," is a paper on the 

 natural scavengers of the coast, in which is stated the 

 curious fact, that at Boulogne the species Nassa is 

 very abundant, and works very hard in destroying 

 dead and decaying animal matter. At Le Portel, 

 a fishermen's village, so near that it is but a 

 suburb to Boulogne, Nassa is scarce, but Eurydice 

 pulchra is very abundant, and transacts the necessary 

 business. At Cape Alprech, there are neither 

 Eurydice nor Nassa, but Ligia oceanica performs ; and 



