102 ANNUAL OF SCIENTIFIC DISCOVERY. 



particularly felt in our crowded towns, where we are obb'ged to be 

 constantly on the lookout for all that is passing around us. 



BREWSTERIAN LIGHT FIGURES. 



Prof, von Kobell, of the University of Munich, -has recently pub- 

 lished an account of some interesting experiments similar to those 

 made by Brewster in 1837, relating to the appearance of bright stars, 

 elliptical colored rings, and various coruscant forms visible, at certain 

 angles, in particular minerals and salts. Pliny was aware of this phe- 

 nomenon, and mentions a stone that he calls Astreos, which showed a 

 bright star reflected within it. The stone he alludes to is, however, be- 

 lieved to be no other than a sapphire, which presents the brilliance de- 

 scribed. 



For a long time, it was believed these appearances were only to be 

 seen in sapphires and garnets, until about the same time as Brewster, 

 Babinet and Volger discovered the incorrectness of that opinion. 



Babinet found that the phenomenon was produced by numberless 

 parallel grooves or furrows, whose sides were all situated at regular 

 angles to each other ; so that the light which fell on the crystal was 

 reflected back from an infinitesimal number of such furrows' sides or 

 little angular slopes. He therefore gave the forms of light thus pro- 

 duced the name of "trellis-work appearances;" and the more simple 

 forms may be produced by engraving on a smooth copperplate the fit- 

 ting parallel lines in close proximity to each other, and then, in a 

 darkened room, holding the plate before a candle in such wise that the 

 reflected light of the flame be transmitted from the plate to the eye. 

 In this case, a streak of light is perceived on the spot where the lines 

 are drawn ; should, however, two sets of lines be drawn on the plate at 

 rirrht angles to (crossing) each other, a streak of light crossing the other 

 bright streak at right angles will be seen. If the lines on the plate 

 the rows of little furrows rather cross each other at an acute angle, 

 then the lines of the bright cross, which becomes visible, are also at an 

 acute angle to each other. Should the drawn lines all emanate from a 

 common centre, then, if the light be transmitted at a certain angle, a 

 parhelion is observed. This last appearance may often be seen in a 

 small piece of glass rod about one-third of an inch thick, when the base 

 of the portion thus cut off is ground smooth. Be it observed, however, 

 that it is not every piece which shows it. By bringing this at a proper 

 distance between the eye and a taper, as above described, a ring of 

 light is seen, in the midst of whose periphery the flame of the candle 

 will be placed. In crystals, a perfectly formed parhelion is very seldom 

 seen. 



Babinet attributed these appearances to the filamentous structure 

 and the corresponding laminated transits of the crystal. Volger called 

 attention to the fact, that very often it is the various faces of a twin 

 formation which cause this coruscation, and that the star-like appear- 

 ance in a ribbed or striped outer crystalline surface changes when the 

 said surface is polished and then again looked through. " But," ob- 

 serves von Kobell, " neither of them makes any mention of the re- 

 searches which Brewster, contemporaneously with Babinet, made in 

 the matter, by experimenting with surfaces naturally corroded, as well 

 as with those whose inner structure was made available for the pur- 



