Sept. 25, 1885.] 



♦ KNOV/LEDGE ♦ 



269 



1 I'v iho Canada 

 he liilfer passes 

 ■(■ ].a,r;illel to the 

 .0 of these prisms 



OPTICAL RECREATIONS. 



By " A Fellow of the Rotal Astronomical Society." 



( Continued frmii j). 198.) 



RESUMING now our description of polarising appa- 

 ratus, we may mention that there are two other 

 forma of it which act by refniction, the Nicol's prism 

 and the double-image iivisni, a very brlff (Ifsi-ri'iit inn of 

 which must suffice 1i.it. Tlir l.n,>,-v cohmsis ,.f a 

 long rhombohedron of Li1:miiI spur ( ul ili:/j-nn;,lly in a 

 particular direction, and with it.s cul f;icrs c.-iiiiiifi d in 

 their original position with Cauad i li;.!-; n. ;.fi.i' iluy 

 have been polished. The effect of tliis :,rr;.n/ri,.. i,t is 

 that a ray of light falling on the prism is splii i,s u.uul 

 into an ordinary' and an extraordinni-^ 

 is reflected out at the side of (lie |ir 

 balsam, and so lost altogether, wliil. 

 through and emerges at the opposite^ 

 incident ray. If light passes through 

 and is viewed at its emersion through 

 the rotation of the latter round the ray as an axis will 

 produce phenomena of the alternation of light and dark- 

 ness identical with those just described in connection 

 with the Tourmaline plates. In all these cases the 

 crystal through which the light first passes is called the 

 polariser ; that through which the polarised beam is 

 viewed, the analyser. The double-image prism men- 

 tioned above consists of two prisms, one of Iceland spar, 

 cut with its optic axis parallel to its refracting edge, and 

 the other of glass having a refracting angle equal to that 

 of the spar. As the ray, passing through the prism of 

 spar, falls on it perpendicular to its optical axis, the two 

 into which it is divided have the greatest pcssible sepa- 

 ration. The glass prism seems to neutralise the colours 

 produced by the crystal one. In this form the extraor- 

 dinary ray occupies the middle of the field, and the 

 ordinarj- ray revolves in a circle round it as the prism 

 rotates round the incident ray as an axi.s. Tliis is a vrry 

 handy form of analyser. It has another us<- altouctlur, 

 to which we shall have occasion to refer wlieu wo ceine 

 to deal with chromatics by-and-by. 







So far we have spoken solely of polarisation by refrac- 

 tion, but rellectiou also is an active agent in its produc- 

 tion. In point of fact, almost all (save metallic) bodies 

 polarise the light they reflect, as may be seen by any 

 one who will look at the surfaces of still water, leaves, a 

 jioli.shcd table or pianoforte, &c., through a plate of 

 Toiu-maliiic or a Nicol's prism. Now it may very well 

 liappeu tiiaf the reader of these lines may, either for 

 economical reasons, or from the fact that he is resident 

 "far from the busy haunts of men" generally, and opticians 

 in particular, be unable to procure either of the simple 



pieces of apparatus which we have endeavoured to make 

 intelligible. For the benefit of such, we have ourselves 

 constructed a simple — not to say rude — form of apparatus 

 for polarising light by reflecti.m f..r the purpose cf 

 description in this papca-, wliieh it needs but the most 

 rudimentary iiie.hinie.il iie.iiiireiin uis to make. Its 

 construction is slmwn in tlie li-iires wliich follow. 



The materials we re.^nire are a tin canister with a 

 lid, such as are used to contain mustard, &c., and some 

 patent plate " used for photographic 



legatives being ■> 

 vhich the sketcli 



iiitable. Oui 



a. h 

 in li t 



diameter 1 



Fig. 49, w: 



We first cut a hole 11 1', i 



the bottom of the canisie, 



hole put a bundle of ha 



clined at an angle G P D 



i I 1 



canister, fro 



ie!d disinfecting- 



ii ' I' I wo turned 



::: -a at A in 

 1.-, being removed. 



■in"- towards thi.s 

 s plates, G P, in- 

 the bottom PU. 



o\it by th: 



from which our illustr.'.t 

 of our plates = 4 in. a 

 2iin. The object of tilt 

 is that light is polari 

 at an angle of 54° 35' from th 

 "' ■ " and refle. 



perpendicular. 

 and into tliis fi: 



way, before fix; 

 the can, the u 

 must, by th 



-,a! ray en term 

 ngle of 35 ' 25' 



; be 

 . the othi 



■eflected up the 

 • side of the 



out of the lid L we 

 nother piece of ourfrlass. ■ 

 -3 blacken at the 1 

 lie glass, we cut slits S 



othe 



;e ef ^ 



thilt o 



lall 1 



with the top of it. 



candle in front 

 the tuljo, with tl 

 by eur n-ur,', an,l 



thei.laekened niin 



