ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 10 5- 



analyser. The colourless lines are produced by the rays for which I 

 vanishes independently of the value of the retardation R, i.e., by the 

 rays for which either sin ia = 0, or sin 2/3 = ; in other words by 

 rays of which one of the planes of vibration is parallel to or perpendicular 

 to the section of a nicol. The condition of parallelism is, however, 

 impossible in a conical beam, and therefore the cone of rays giving the 

 colourless line is the locus of rays of which one of the planes of vibration 

 is perpendicular to the section of a nicol. The author follows up three 

 modes of investigation and arrives at an equation (in general of the 

 third degree) for the colourless cone ; and at an equation (in general of 

 the sixth degree) for the refracted cone. 



In a later article * the author points out that the fundamental 

 formula implicitly supposes that the vibration of the beam emergent 

 from the polariser has not been deviated in traversing the lens which, 

 renders it convergent. This is not exact, both by reason of the 

 deviation given by the glass to the vibrations oblique to the planes of 

 incidence, and, d priori, by reason of the obliquity of the rays them- 

 selves. He therefore reconsiders the question on the basis that the 

 horizontal deviation of a ray must be negligible. The colourless cone 

 thus becomes the locus of directions of propagation, possessing a direction 

 of vibration parallel to the section of a nicol. The author was some- 

 what surprised to find that he arrived at his previous equations, the 

 explanation being that, if a direction of propagation possess a vibration 

 parallel to the section of a iiicol, the plane of vibration which corresponds 

 to its other vibration is normal to the section of the same nicol. 



Lowe, F. — Ein neuer Spektrograph fiir sichtbares und ultraviolettes Licht. 



[This is a description of Pulfrich's auto-collimation-spectroscope, made by 

 C. Zeiss.] Zeit. f. Instrumentenk., xxvi. (1906) pp. 330-3 (5 figs.). 



(6) Miscellaneous. 



Fluid Crystals.!— Under the title of "Are Crystals Alive," E. E. F. 

 gives an account of a communication which was made by 0. Lehmann 

 at the last Congress of German Physicians and Physicists at Stuttgart. f 

 It refers to some new and striking analogies between the development 

 and characteristics of crystals and those of the lowest living organisms, 

 and demonstrates the fact that no hard and fast line of demarcation can 

 be drawn. This has been suspected by Haeckel for some time past. 

 That ice-crystals imitate vegetable forms is known to every child. That 

 they grow we all know. They have also a certain recuperative power, 

 and they require a nucleus or germ to start their growth. They have, 

 in addition, a power of absorbing foreign substances, as when salam- 

 moniac crysUils absorb chloride of iron from a solution, and become 

 darker than the solution itself. In the course of the process they 

 " poison " themselves, and their growth becomes very irregular and 

 imperfect. 



But one essential difference remains. Animals are semifluid, or 



* Bull de I'Acad. roy. de Belgique (Classe des Sciences) 1906, pp. 493-502 (1 fig.), 

 t Englisli Mechanic, Ixxxiv. (1906) p. 371. See also Zeitschr. wiss. Mikrosk.,, 

 xxiii. (1906) pp. 377-9. 



X Physikal. Zeitschr., Nov. 1, 1906. 



