HARD WICKE ' S S CIENCE ■ G OSSIF. 



Ill 



that produces the six-rayed star has its crystals dis- 

 posed mainly in three directions. 



Very often a six-rayed specimen will give six other 

 intennediate and less brilliant rays. These secondary 

 rays are seen to be produced by more minute or less 

 frequent ciystals, arranged at intermediate angles 

 with those crystals that form the primary and brighter 

 star. Occasionally a piece will show two or four 

 more rays, making the star look unsymmetrical. 

 Careful inspection will then show other sets of crystals 

 at a different angle. 



The crystals vary in shape, sometimes being short, 

 flat, and tabular, but are usually very narrow, being 

 about from six to ten times longer than their 

 width. 



They have often a faint, pinky-blue colour, though 

 the majority are colourless ; occasionally a yellowish 

 crystal may be seen. 



The larger crystals show polarization pretty well. 

 Numerous minute " Newton's rings" are formed by 

 the excessively thin laminae, of which even the thin- 

 nest film that one can prepare is composed. These 

 are best seen v.-hen the film is mounted in Canada 

 balsam. Prolonged soaking of pieces in ether, and, 

 after-immersion in turpentine, successive exhaustions 

 by the air-pump, failed to exhaust the air entirely from 

 between these exceedingly minute laminae. It oc- 

 cuiTed to me that a film might, perhaps, give some 

 effects with polarized objects if used above or below 

 the eyepiece of the microscope ; but I failed to find 

 that it gave any at all. Its action on light can be 

 well imitated by blacking a plain microscopic slide 

 over the flame of a lamp, and then with a camel-hair 

 brush scratching fine lines along the slide. When a 

 light is looked at through this, a ray of light is seen 

 crossing the flame at right angles to the lines scratched. 

 Now, on cross-hatching these with lines at right 

 angles, and again looking at the light, a luminous 

 cross is observed. Then, if lines at angles of eighteen 

 degrees with the former lines be made (not an easy 

 task), a twelve-rayed star may be seen, on viewing a 

 light through the glass. 



This effect may be, however, far more easily and 

 brilliantly produced by spreading a little viscid oil on 

 a large microscopic cover-glass. The strite in this 

 case are made by wiping off the mass of the oil by a 

 single straight rub, using a coarse, napless cloth. 

 This will give, as in the case of the partly-blackened 

 glass, a brilliant ray at right angles to the strire. By 

 single careful rubs two other sets of lines may be made 

 on the same side of the glass, producing a six-rayed 

 star. Now, the other side of the glass is to be treated 

 similarly, taking care not to smear the first side, and 

 making the rubs at alternate angles to the last set. 

 Thus, by very careful manipulation, a twelve-rayed 

 star may be produced. A glass slide does not succeed 

 so well as a thin cover-glass, its thickness preventing 

 the striae on opposite sides of the glass focussing in 

 the eye at the same time. 



A familiar instance of the same phenomenon must 

 have been observed by most people. 



Coming along the streets on a rainy evening (no 

 rarity of late), if you are under a silk umbrella and 

 chance to twirl it round as you approach a street 

 lamp, you may have noticed a luminous ray extending 

 vertically down the middle of a gore. As a fib 



Fig. 103. Slide of Canadian Phlogopite, showing confused 

 appearance of Crystals x 250 diam. 



Fig. 104. Another Slide of ditto, showing tendency to rayed 

 condition x 250 diam. 



Fig. 105. Diagram illustrating radiating "gores" of an 

 umbrella. 



approaches toward the liglit, the ray is inclined, say, 

 from light to left ; on the rib passing the light, and 

 the edge of the next gore coming in front of the lamp. 



