Au^. a 2, 1872] 



NATURE 



333 



Mr. Si'orriswooDE's Lecture to Working Mem on Sun- 

 light, Sea, and Sky 



TuERK are mrmy ways in wliicli men have looked at life, tlie 

 higher kind of life, that iJeil which each of us f jrms in his 

 own mind lo which we each hope that we are always tending. 

 But aU the.se various ideas may for the most part be grouped 

 under two heads: the Ideal of Rest, and the Ideal of Work, 

 " Rest, rest ! " said a brave old Germiii worker, "shall I not 

 have Kternity to rest in ? ' That represents one view. " Work, 

 work!" Slid another, "must I not work now, that I may the 

 better «-ork in Eternal Life ? " That represents the other. But 

 without entering upon the somewhat transcendental question of 

 a future life, these ideas and aspirations have a meaning and 

 realty even in the life which we now live. How do we hope to 

 spend the leisure which old age may some i\y bring? Or, 

 nearer still, when the day's work is dune, and the day itself is 

 not quite spent ; or when such holiday as miy befall each of us 

 comes round, how do we hope to spend the time? Do we long 

 for mere rest, for that 



Do we desire to sit us 



Kind 

 ned :\l\\My5 .ifterno 



upon the yellow sand 

 and muon upon the shore, 



and sing with the lotus eaters 



All things have rest ; why should we t-.il alon 

 Nor steep our brows in slumber's holy balm, 

 Nor hearken what the inner spirit sin^ji. 

 There is no joy but calm. 



Or do we rather with Ulysses say. 



How dull to p.ause, to mike an (nd 



To rust unburnished, not to shine in use! 



life. Life piled on life 



Ti 



i.tle 

 that 



but 

 irnal silen 



:thmg 



; saved 



A bringer of new things ; and vile 



For some [few] suns to store and hoard myself. 



And this gray spirit yearning in desire 



To fo'low knowledge like a smking star 



Beyond the utmost bounds of human thought. 



To which of these two ideals I myself lean has jierhaps already 

 betrayed itself; and that being so, I shall venture to consider 

 your presence here a proof that, for tliis evening at least, you 

 sile with me, and that you are willing to spend an hour of your 

 lci:>ure in an intellectual effort to see a little deeper into those 

 phenomena which Nature in this place .and at this season dis- 

 plays with such profusion and splendour. 



But at the outset I must warn you that we are met by a diffi- 

 culty, for the surmounting of which you must rely upon your- 

 selves rather than upon me. It is this : the phenomena to which 

 I propose to draw your attention, allhough taking place nearly 

 every day, and all day long, and in almost every direction, are 

 veiled from our e)es; and it is only by the use of special 

 appliances to aid our eyes that they can be made visible. It will 

 be my bu-iness to supply these appliances, and, repro:lucing on 

 such scale as may be possible within these four walls the optical 

 processes which are going on in the sea and sky outside, to 

 exhibit the hidden phenomena of which I am speaking. But it 

 must be your part to transport yourselves mentally from the 

 mechanism of the lec.ure-room to the operations of Nature, and 

 by a "scientific use of the imagination" (to adopt what has now 

 become a household word at these mee'ings) to coimect the one 

 with the other. 



Now the main point in questi.m is this : that light, when sub- 

 jected to the very ordinary processes of reflexion from smooth 

 surfaces, such as a window, a mahogany table, or the sea itself, 

 or when scattered to us from the deep clear sky, undergoes in 

 many cases some very peculiar changes, the character and causes 

 of which we have come here to investigate. The principal 

 appliance which will be used to detect the existence of such 

 changes, as well as to examine their nature, consists of this piece 

 of Iceland sp.ir, called — from the man who first constructed a 

 compou'id block of the kind — a Nicol's prism, and this plate of 

 quartz or rock ci^stal ; both of which, as you will observe when 

 the light passes through them, are clear, transparent, and colour- 

 less, and both of which transmit the direct light from the electric 

 lamp with equal facility, however they may be turned round 

 about the beam of light as an axis. 



If, however, instead of allowing the beam to fall directly upon 

 the Nicol, we first cause it to be reflected from this plate of glass, 

 we shall find that the process of reflexion has put the light into 

 a new condition. The light is no longer indifferent to the rota- 

 lion of the Nicol ; in one position of the Nicol the light passes 

 as bffore, but as the instrument is turned round the light gradu- 

 ally fades, and when it is turned through a right angle the light 

 is extinguished. B-yond this position the light reappears, and 

 the same changes of fading and revival are observed in the light 

 for every right anv:Ie through which the instrument is turned. 



But the-e phenomena are susceptible of a very beautiful modi- 

 fication by the interposition of this plate of quartz between the 

 rcflectnig .surface and the Nicol. The changes in the light are 

 no longer mere alterations of brightness, but exhibit a succession 

 of colours resembling in their main features those of the rainbow 

 or .specirum. 



The pcculi.ir condition to which light must be brought in order 

 that these phenomena may be produced is called polarisation ; 

 and although an explrnation of its nature must be reserved until 

 later, I beg you to notice that it is effected in this instance by re- 

 flexion fnmia plate of glass. A similar effect is produced if hght be 

 reflected from many oiher substances, such a- the leaves of trees, 

 particularly ivy, mahogany furniture, windows, shutters, and 

 o'len roofs of houses, oil paintings, &c., and last, but not least, 

 the surface of water. In each of these cases the alternations of 

 light and darkness are most .strongly marked, and the colours (if 

 a quartz plate be used) are most vivid, or, in technical language, 

 the polarisation is most complete, when the light is reflected from 

 each substance at a particul.tr angle. In proportion as the m- 

 clination of the light deviates from this angle the colours become 

 fiinter, until, when it deviates very greatly, all trace of polarisa- 

 ii i\\ at last disippe.irs Without occupying the time necessary 

 to shift our apparatus .so as to exhibit this with the glass plate, 

 we may aher the reflecting surface from glass to water, and by 

 projectuig on the screen the beautiful phenomena of liquid waves, 

 make visible the difl'crent degrees of polarisation produced at the 

 variously inclined poitiims of the surfaces of those waves. A 

 tea-tray will serve as well as anything else to form our little sea, 

 and a periodic tap at one corner will cause ripple enough for our 

 present purp ise. The «'aves now appear bright on the screen, 

 and although brighter in some parts than in others, they are no- 

 where entirely dark. But; on turning round the Nicol the con- 

 trast of light and darkness becomes much stronger than before. 

 Here and there the light is absolutely extinguished; in these 

 parts the polarisation is comple'e. in others incomplete in v.arious 

 degrees- And if the quartz plate be again introduced we have 

 the beautiful phenomena of iiis-coloured rings playing over the 

 surface of our miniature sea. 



Now, that which you see here produced by our lamp and tea- 

 tray, you may see any day under the bright sky of this southern 

 coast. By using an appar.itus such as we have here, or a simpler 

 one which I will immediately describe, you m.ay bring out for 

 yourselves these phenomena of colour, and thereby detect the 

 prolusion of polarisation which Nature sheds around us. But 

 before describing it, there is one peculiar feature of all these 

 experiments which must be noticed — namely, that the same 

 results would be produced if we changed the positions of the 

 lamp and the screen. The light which is now polarised by the 

 glass or the water, and examined by the Nicol, might equally 

 well be polarised by the Nicol and examined by the glass or the 

 water. And, therefore, if we find that any contrivance will 

 serve for the one purpose, we may conclude that it wdl serve 

 equally well for the other. 



And now a word about that simpler apparatus. When light 

 falls upon a transparent substance, part is reflected, part trans- 

 mitted. If, therefore, the reflected part is polarised (and you 

 have already seen that this is sometimes the case), it is not sur- 

 prising tint the transmitted part should be so also. And further, 

 if the polarisation by a single reflexion or transmission is incom- 

 plete, it will become more and more complete by a repetition of 

 the processes. "Ihis being so, if we take a pile of glass plates — 

 say half a-dozen, more or le.ss, the thinner the better — and hold 

 them obliquely before our eye at an angle of about 30° (say one- 

 third of a right angle) to the direction in which we are looking, 

 we shall have all that is necessary to detect the presence of 

 polarisation ; a'ld if, lurlher, we hold a piece of talc or mica, 

 such as is commonly Used as a cover to the globes of gas-burners, 

 beyond the pile of plates, colour will be produced in the same 

 general manner as with the q'larlz, although with some essential 

 difference in detail. 



