March 15, 1900] 



NATURE 



465 



"varieties" the Streptococcus erysipelatos, the Strepto- 

 coccus conglomeratus (Streptoc. scarlatinre), the Strepto- 

 coccus brevis and longus, Streptococcus murisepticus, 

 and Streptococcus septo-pya?micus. According to the 

 author, the differences in size, arrangement, cultural 

 characters and physiological action of these "varieties" 

 and the " Streptococcus pyogenes " are slight, and do not 

 justify a separation as true species. Now, any one who 

 has had sufficient experience in the matter of these so- 

 called " varieties " must know that the cultural and 

 physiological differences between these " varieties " and 

 the "species "' are sufficiently definite and conspicuous ; 

 in fact, quite as definite as those described of several 

 others of the author's true "species" of Streptococcus. 



The same difficulty is met with in looking over some 

 of the species of the genus Micrococcus, Bacterium and 

 Bacillus. As mentioned above, the chief distinction be- 

 tween genus Bacterium and Bacillus is the absence or 

 presence of flagella ; now looking through the description 

 of some of the species belonging to " Bacterium," we find 

 several in which the absence of flagella is deduced 

 apparently solely from the fact that in the fresh state 

 (hanging drop) no mobility is observed ; but this, as is 

 well known, is deceptive for a true diagnosis, and no safe 

 reliance can be placed on it. In the same way we find 

 some species of " bacillus," e.g. bacillus pestis, as being 

 surrounded by flagella. I have no doubt this statement 

 will come to many as a surprise, and one would like to 

 know whether this bacillus pestis of Migula had been 

 tested on animals and had caused the typical disease. 



The volume contains at the end eighteen plates, each 

 with eight figures of clear and good prints of photo- 

 graphic representations of many species of Coccacese, 

 Bacteriace^e and Spirillace?e. Many of the figures are 

 excellent, e.g. those of Flagellate bacilli, Pseudomonas and 

 SpirillaceiE ; some others might without disadvantage 

 have been omitted as not representative or too little 

 representative ; e.g. there occur five figures of Vibrio 

 cholerac asiaticae [Microspira Comma (Migula)], not 

 one of which is really characteristic of the microbe. 



The important points of the formation, appearance and 

 distributions of spores in many bacillary species, is repre- 

 sented by a single figure (Fig. 2, Plate iv.) showing dots 

 in anthrax threads supposed to have been photographed 

 at a magnification of 1000 (1). 



The book on the whole must occupy an important 

 place not only as a thoroughly systematic work, but also 

 as a book of reference, there being attached to each 

 species a valuable paragraph of bibliography. 



E. Klein. 



COLLECTED WORKS OF L. LORENZ. 

 Ouvres Scientifiqties de L. Laretis. Retmes et Annoi^es. 

 Par H, Valentiner. Tome Premier, Deuxi^me Fas- 

 cicule ; Tome Second, Premier Fascicule. Pp. 2 13 -I- 529 

 and 315. (Copenhague : Lehmann and Stage, 1898 

 and 1899.) 



THE custom of collecting into convenient form the 

 works of a distinguished writer has much to recom- 

 mend it. We in England have realised its importance, 

 and we gladly welcome this edition of the collected works 

 NO, 1585, VOL. 61] 



of Prof. L. Lorenz, two parts of which are now before us, 

 published in French, at Copenhagen, under the editorship 

 of Dr. H. Valentiner, and at the cost of the Carlsberg 

 Foundation. The two volumes cover a wide period of 

 time ; the first paper, that containing Prof. Lorenz's 

 theoretical and experimental researches on indices of 

 refraction, was printed in 1869. The author's name is 

 well known as one who has worked at optical theory, and 

 has carried out experiments of great importance with a 

 view to the verification of crucial points in that theory. 

 The phenomena of dispersion, and the relations between 

 the optical properties and the physical conditions of a 

 substance, offer a fascinating field of research ; and it is of 

 real service to have here, in accessible form, the elaborate 

 series of papers which led Lorenz to the conclusion that 

 the quantity (/x^ - i )/(/ii- -f- 2)p was a constant for the various 

 states of a refracting medium. This is hardly the place 

 to discuss at length the various steps that lead the author 

 to that conclusion In Lorenz's view the ether inside a 

 transparent medium, such as glass or water, cannot be 

 treated as homogeneous. His solution of the problem is 

 most easily followed in the paper, " Ueber die Refractions 

 Constante" {Wied. Ann. tome xi.), the mathematical de- 

 velopments of which are given on p. 360 of the first 

 volume now under consideration. Lorenz assumes, in 

 this paper, that within the molecules of a transparent 

 body the velocity of light is constant, and in the inter- 

 spaces between the molecules it is also constant ; the 

 actually observed velocity will depend on these 

 two constants. In the paper now before us it is 

 assumed further, though this is shown not to be vital 

 to the result, that the molecules are spheres. The 

 problem thus discussed is that of the transmission of 

 light through a complex medium consisting of trans- 

 parent spheres embedded in a homogeneous medium, 

 and with these assumptions it is shown that the 

 quantity (/^co'*- 0/(Moo^ + 2) is proportional to the mass 

 per unit volume of the compound medium. In obtain- 

 ing the above equation, the effects of dispersion are 

 neglected ; a later paper ( IVied. Ann. tome xx.) dis- 

 cusses these on the assumptions (i) that the density of 

 the ether near any molecule is a function of the distance 

 from the centre of the molecule, so that the ether is 

 arranged round each molecule in spherical layers, which 

 change in density on passing from one layer to the 

 next ; and (2) that Fresnel's sine and tangent formulae 

 hold for each such transition. 



From this Lorenz obtains the equation 



(M-'^-M»-);(M- + 2;x„> = rt/X2-hA/X^-h . . . 



;x being the refractive index for waves of length X, and 

 /[i„ that for infinite waves. 



Other papers in the volume before us are concerned 

 with experimental investigations into the truth of these 

 formulae. As a result of one series of experiments, it 

 appears probable (p. 245) that the refractive index of 

 water is a function of the density of the water, and not 

 of the temperature, except so far as that produces change 

 of density ; while, in general, Lorenz concludes that for 

 a number of gases and vapours the equation 



(Mm-- I )/(/*« - + 2)p=rt constant 



is satisfied with considerable accuracy. 



