ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 057 



be immersed some minutes in a solution of 70 p.c. alcohol 200 c.cm., 

 Mars 1 yellow (aniline) 1 grm. When the plate has been dried in a 

 dustless atmosphere it will be found to have a tint approaching the 

 ideal yellow. Other aniline colours after comparative trials with the 

 spectroscope will give by this process excellent screens of other tints. 



(5) Microscopical Optics and Manipulation. 



Behaviour of Crystals in Light Parallel to an Optic Axis.* — 

 If a section of a biaxial crystal be cut normal to an optic axis, and this 

 section be examined in parallel light between crossed nicols, it appears 

 uniformly bright in all positions when rotated about the axis. C. Travis 

 points out that this phenomenon is commonly ascribed to interior conical 

 refraction ; but he considers that, owing to the neglect of certain im- 

 portant factors, this conclusion is untenable. The object of his paper 

 is, therefore, to present a discussion of the behaviour of crystals in light 

 that is approximately parallel to an optic axis, and to explain the 

 observed differences between uniaxial and biaxial crystals under these 

 conditions. His conclusions are : 1. That interior conical refraction, in 

 a strict sense, plays no part whatever as a cause of the phenomenon. 

 2. That the cause is to be found in the fact that so-called parallel light 

 has commonly a considerable divergence. 3. In any given case, the 

 observed intensity of illumination is equal to the average intensity of 

 that portion of the interference figure bounded by the limits of the 

 pencil of light used. The general configuration of the interference 

 figure is dependent upon the optical constants of the crystal, and upon 

 the thickness of section ; these, as well as the amount of divergence of 

 the light, are the determining factors. 4. That the reason why the 

 same phenomenon is not commonly observed in uniaxial crystals is that 

 in the uniaxial figure the first bright ring about the axis is iu general 

 much larger than that in the biaxial figure. Under proper conditions, 

 however, the phenomenon may be also shown by a uniaxial crystal. 



In the course of his paper he gives the demonstration illustrated in 

 figs. 95 and 1)6. A ray S A (fig. 95), from a source S, is divided upon 

 entering a biaxial crystal, into the rays A B and A C, which vibrate at 

 right angles. From the same source another ray, S D, may be found 

 which will divide into D C and DB: C is then the common point of 

 emergence of one ray from each of the points A and D. These rays are 

 polarized at right angles. If the crystal is between crossed nicols, inter- 

 ference takes place between the components of A C and A D parallel to 

 the plane of the upper nicol. The effect produced is dependent upon 

 the difference in phase at C, and this is due to the difference in the 

 optical length of the paths S A C and SDC. If the wave-front of the 

 two rays is essentially normal to the optic axis, it can be shown that 

 the path SAC (S A being great) is optically equivalent to S D 0. If, 

 then, S is at a great distance the two rays at C will be in phase ; their 

 vibrations will give a resultant which is parallel to the plane of the 

 lower nicol, and this resultant will be extinguished by the upper 

 nicol. 



* Amer. Journ. Sci., xxix. (1910) pp. 427-34 (2 figs.). 



