POlARISiTIOJI COLOUR. ] 



UNDULATORY FORCES. LIGHT. 



83 



such qualities of bodies, and to give us an insight into 

 other causes than those which we assign to many effects 

 observed in daily life. We should, perhaps, trench too 

 greatly on the doctrines of chemical science, were we to 

 fully express our views on this subject, and illustrate 

 them by the countless instances which we could adduce, 

 wherein a change in the molecular constitution of a body 

 modifies its character. We may, however, take the 

 case of alcohol, and its products. Retaining the same 

 elements alcohol, ether, acetic acid, oxalic acid, <tc., 

 are all producible from one substance ; namely, common 

 sugar. By means of what has been termed catalysis, 

 alcohol is easily converted into acetic acid ; and yet the 

 only agent required is one platina, which does not 

 undergo the least perceptible change during the process : 

 similarly, the same metal can effect the union of oxygen 

 and hydrogen, especially when presented to a mixture of 

 those gases when it is in a state of minute division. 

 Instances of the rearrangement of atoms (purely physical 

 in its character) may be adduced, wherein no chemical 

 change is perceived, no external alteration made mani- 

 fest ; and yet no sooner do we employ a ray of polarised 

 light, than we are enabled to detect differences and 

 effects which otherwise would entirely escape our senses. 

 Annealed and unaunealed glass, Iceland spar, and 

 Arragonite, the different kinds of oils, <tc., are illustra- 

 tions of such occurrences. We must, however, not stop 

 at a general view of the matter : our desire has been to 

 give the student a distant but comprehensive view of 

 Iraost unlimited nature of our subject ; and having 

 attempted this, we shall endeavour to lead him, step by 

 step, among the arcana of this interesting branch of 

 physical research. 



We shall presume that the student has provided him- 

 self with some form of polariscope such as we have already 

 described ; or, in the absence of this, the following will be 

 necessary ; namely, two bundles of glass plates, each 

 perfectly clean, and to the number of ten in each bundle. 

 These should be fastened together, as illustrated pre- 

 viously.* A large piece of plate or window-glass, which 

 has been blackened on ono side, either by lampblack or 

 the black varnish used by photographers, and a few 

 pieces of mica, on which to try various experiments, 

 will be required. The latter may bo obtained in most 

 of our large towns, as it is largely employed for the pur- 

 pose of making lamp-protectors, and for other purposes. 



Furnished with these, our readers may readily study 

 all the leading phenomena of polarised light. Indeed, 

 the whole of the results wo shall for the present refer 

 to, may be produced by the simple arrangement we 

 have named. This is another illustration of the fact, 

 that science, even in its most refined branches, may be 

 experimentally studied at a trifling cost. 



Having placed the polarising black plate with the 

 polished surface upwards, so as to receive light thereon 

 from a white cloud, or from a lamp screened by means 

 if a piece of ground glass, its surface may be viewed 

 through the bundle of glass plates, or by means of 

 another piece of blackened glass placed above it. If 

 the latter be employed, the polariser and it must be at 

 similar angles (about 57) with the horizon. The angle 

 at which the bundle of glass plates is to be held, de- 

 l-nils on their number. Their position, however, has 

 already been illustrated, t The observer will find, that 

 if cither the bundle of glass plates or blackened 

 analyser be held in the proper position, then the light 

 passing from the polariser will alternately bo visible or be 

 shut off, as the analyser is turned through each quarter 

 of a circle; so that through SCO", there will be, alter- 

 nately, a phase of light, and one in which the light 

 entirely disappears. 



As we have already remarked, this is due to the 



peculiar change which light undergoes when it suffers 



repeated reflection, refraction, or double refraction. 



Tli<! result is obtained by reflection simply when the 



r iscr and analyser are two pieces of blackened plate, 



KiK.h as wo have described. The result occurs, partly 



by reflection and by single refraction, when the 



8t ante, p. 81, t\t. 78. f See aitt, p. 81, Fig. 74. 



polariser is a blackened glass plate, and the analyser a 

 bundle of plates. The same result is arrived at by 

 single refraction solely when two bundles of glass 

 plates are used, placed at the proper angles with each 

 other. When, however, a piece of Iceland spar, or 

 tourmaline, is employed, then these crystals produce the 

 same result by double refraction only. 



As a matter of convenience (although it involves 

 expense), we should recommend those of our readers 

 who can afford it, to purchase a Nicol's prism, which 

 is merely a rhomb of Iceland spar, divided and 

 rejoined in a wedge-like form, for the purpose of re- 

 moving, as far as possible, the extraordinary and ordi- 

 nary rays from each other. The angle of vision 

 through it is of course always the same, its optical 

 character having been attended to during its construc- 

 tion by the optician. It forms the best kind of ana- 

 lyser. It is also desirable that the student should 

 possess two pieces of tourmaline, as they are the readiest 

 means of observing several of the phenomena which we 

 shall mention. In the absence of these, however, the 

 simple apparatus we have named will answer the pur- 

 pose ; and will have also the ad vantage of calling out both 

 the skill, patience, and close attention of the inquirer. 



If pieces of mica, of varying thickness, are interposed 

 between the analyser and the polariser, they will pre^ 

 sent appearances of colour which vary according to the 

 thickness of the substance. Those comparatively thick 

 afford red ; and the thinner kinds give tints of various 

 shades of blue : all the intermediate colours of the 

 spectrum, and their numerous combinations, being thus 

 obtained, their complementaries come in view on the 

 analyser being moved through an angle of 90 with its 

 previous position in reference to the polariser. If, 

 however, the mica bo slightly shifted, the play of 

 colours will be continually varied ; because other planes 

 or laminm come within the lino of view. 



Now these colours correspond in all respects as to 

 order, <tc. (excepting the thickness of plates producing 

 them), with those obtained by means of a lens pressed 

 on glass, the prism, the soap-bubble, &c., on which we 

 have already dilated. { Their immediate cause, how- 

 ever, is polarisation ; whilst those we have referred to 

 previously, are the result of interferences, produced 

 either by simple reflection or refraction. However, the 

 doctrine of the interference of light, is equally appli- 

 cable to that which has been polarised as to light in its 

 ordinary state ; and pursuing the plan we have hitherto 

 adopted, we shall now only refer to our remarks on in- 

 terference^ leaving our readers to extend such in their 

 application to the subject with which wo are now deal- 

 ing. The experimental proofs and mathematical inves- 

 tigations which would be required to fully elucidate this 

 subject of polarised interference, are at once too delicate 

 and abstruse for most of those for whom our work is 

 intended. The advanced physicist may consult, with 

 advantage, papers by Fresnel, Arago, Herschel, and 

 others ; of which we regret to say that no general 

 synopsis exists a fact partly due to the narrow limits 

 in which the study of the subject is confined by its 

 abstract character ; and on account of the necessity of 

 high mathematical attainments on the part of those who 

 would fully investigate it. 



We shall now pass on to consider some magnificent 

 appearances wliich certain substances present when ex- 

 amined by means of polarised light : but, before doing so, 

 we may just name, that the coloured phenomena to 

 which we have just alluded may be obtained in selenite, 

 the yellow prussiate of potass, <tc. ; all of which, like 

 mica, vary in the tints they produce, in proportion to 

 their thickness. Selonite, which is a crystallised form 

 of sulphate of lime (plaster of Paris), is frequently 

 arranged in the form of animals, church windows, <tc. ; 

 and this affords a pleasing and popular illustration of 

 polarised light and its effects. 



Referring to our remarks at a previous page,|| whilst 

 speaking of double refraction and optic axes, wo noticed 

 that some crystals are uniaxial, and others biaxial. The 

 t Sec antr, p. 91. i Ante, p. 51. || 4t,te, p. 79. 



