TO THE MICROSCOPE. 278 



certain structures occurring in the elytra of beetles, such as the 

 cockchafer and large water-beetle, the bordered pits in a section of 

 deal wood, etc. If tartaric acid — or, better, salicine — is properly 

 crystallised on a slide, the crystals will radiate outwards from one 

 or more common centres, and a very fine black cross will be seen 

 under the polariscope. 



This black cross is due to the fact that a doubly-refracting 

 object exhibits no polariscopic appearances when it is placed with 

 either of its two optic axes parallel to the direction in which the 

 light is polarised. Consider, for example, a stellate hair formed 

 by a number of hairs rotating from a common centre (Fig. 7). 

 Let the light be polarised in the direction of the arrow. The two 

 hairs (/, /) are pointing parallel to this direction, and the hairs 

 ( C\ U) are perpendicular to the same direction. Each of these 

 four hairs has one optic axis parallel and the other perpendicular to 

 the direction of polarisation ; therefore, all four hairs appear dark 

 on the dark ground. The four hairs (^, E^ O, O) have their optic 

 axes inclined at 45*^ with the direction of polarisation, and they 

 are therefore in the most favourable position to show polariscopic 

 appearances. The intermediate hairs also show polariscopic effects, 

 which are, however, rather less marked. 



The spines of certain Echinodermata are sometimes mounted 

 arranged in a radiating pattern, and the same effect is well shown 

 in such a slide, notably in " professional " mounts of the anchors 

 and plates of Synapta. 



When a number of plant-hairs combine to form a scale — as in 

 the ElcEagnacecB, above mentioned — the effect is to give a distinct 

 cross. The same is true of the crystals of salicine. Here one of 

 the optic axes of each crystal points towards the centre, from 

 which they radiate, and the black cross is formed by those 

 crystals, in which this optic axis is either parallel or perpendicular 

 to the direction of polarisation. 



A starch-grain is built up of concentric layers, which are gra- 

 dually added to it as it grows. A perfectly round starch-grain 

 would grow directly outwards from its centre in just the same way 

 as the salicine crystals grow. It would, therefore, exhibit a perfect 

 cross in the form of two black straight lines intersecting at right 

 angles in the centre of the grain. As a matter of fact, however, 



International Journal of Microscohy and Natural Science. 



Third Series. Vol. III. t 



