The sun., 1872-12-23
two plates Of glass instead Of one we augment
the quantity OF the

tne quantity OF
TRANSMITTED PorARTZED LIGHT,
and by taking n bundle OF plates we so increase
the quantity as TO render the transmitted beam
for all practical purposes perfectly polarhed.
poSed a. Means :" Of pSsSe sLH eS
Let us push our reasonlnes all further. whom
the tourmalines are crossed, the space where
seen that tho least obliquity ON the part Of the
crystals permits leht to get through both, Now
We interpose third plate OF tournane be-
tween them: with ,ts axis obiq9c to both; %
plate will get through this intermediate one:
Bnt after i has got through, It's plane OF
yibra;lo.n has ch,rged II is no longsfT
ctystal. Thus, by reasoning, we infer that tho
win In purt abolish the darkness produced by
the crossing of the other two platvs have not
wb y"m % cmploy In Sn'ir M stoves l. am more re en-
yenent substance which aris In tue same way.
netween the crossed tyurralnes, INTRODUCE
nlm slowly descending, aud ss II desCends be-
tween the tourmallnes,
LIGrT TAHrs TIM rr.tcr or rAHKSEss.
The darkness. IN fact, seems scrapcd away Rs
II i were something material.
We now stand upon the thrcs hold ~ a new
and splendid optical domain. You have seen
the two beams emcr9cnt from Iceland spar. aud
have pr we'd them % hc polarl,d. II We could
abolish one NJ these beams, We might cWUlOy
the other for experiments on polarzcd Hight.
These bcau;a, rs you know, are refracted differ
entity, and from this We are able to infer thut
under son'c circumst tan"cs the one may he to-
tally reelected, and the other not. An Ortician
named Nicol cut n crystal uf Iceland spar In
two IN Il certain dire ction. lie polished tho SOW
cred surfaces and reunited them by Canada bal
sam the surface OF union being So inclined u
the Beam travcrsing the spar that the ordinary
tally reflected by the balsam. while the extraor-
dinary ray was permitted to pass on. The invcn-
ton Of tho Nic"l prism was great step IN
practical optics, and quite recently such uttatns
have been constructed OF "A size which enables
AUDIENCES IKE TuT PRESENT
| to wi'ness the Chr'matic phenomena of polar
| short time ago. The two risms here tciurC you
| h"lnng 70 my excellent friend. VII Willt.m sl1t-
| ladd. ] have With me another pair uf very
| noole prl>ms, SUI laLgcr thn these, m"nutn,
/ 50 high and wcll-meritcd reputation IN the
| construction of spectroscopes.
These two Nlco prisrns play the same part hs
| the crystals Of tOurmalne. Placed with their
| directions cfVibraton parallel, the light passcs 4
| through both. When these directions are cross :
| ed the hght - quencbed. introducing a nlm ~
| mica between the pri-nls, the light Is IN part
| restored But notice, when the hlm OF MICA
Is thin, you have not only light, but
come will be the examination of these Colors.
With this Wow, will take n cntatiyc crNg
| tai, one easily dealt with; thar ~, the crystal
| gypsum, or selenite, which IL crystnlliIed sui
| phaie Of lime Between the crossed Nicols
place thick plate OF this cry Stal like the mica,
| L restores the Light, but II produces NO color.
| With INV penknife take a thin splinter from
this crystal and place u between the pristnS II
| image on the screen glows with the richest
| colors. Turning the prism In front these colurs
| gradually fade. disappear. but by cOntinu n q
rotation until the vibrating sections OF the
| pTisuis are parallel,
| VIVID CODoRs AGAIN APPEAR,
| bit these colpfs are Complementary 70 tie
fortuer ones.
| some pa'thes Of the splinter appear ~ one
| color, some OF another, these dintrt ncc~ being
dre to the different thiCkne,scs of the tlm. II
| the thickness bc uniform, the co.or is uniform
| Here. for Instance, IL a stellar shape, every
| loZenge Of the star being nm uf gypsum ui
| uniform thickness. Each luZeuge, Vou observe
| shows a brilliant uniform color. OF course II
is easy. 10y shaping our Bms so ~ 10 repres"ni
| flowers ot other objects, 90 exhibit such object,
in colors unattainable 10y art. Here. for eX'
ample. II a specimen uf heartsease, the colon ui
which YOU might safely defy the artist to repro
| duce. Nv turning the front Nicol A deg. round
we pass through n colorless phase to series p
coors Complementary t. the former ones
| Here. for example, Is rose tree with red
flower and green leaves. turning the prism
ninety degrees round, We obtain green howi
and red leaves. All these wonderful chrnmatit
Effects have definite mechanical causes in tn,
! m ntions Il the ether the Ir nctplc NJ interter
ouUe. daily applied and Interpreted, explain,
them all and II you give me your pHtience. ",
sall, - far a. IL IT, M, essaTy, develop the cause
OF these effects In our ncAt lecture.