TRANSACTIONS OF THE SECTIONS. 41 



case of refloction from iron, as in most cases of transmission through salts of iron, 

 the plane of polarization is turned in a direction contrary to that of the magne- 

 tizing current. 



A Description of Sjjottiswoode's Poclcet Polarizing Apparatus. By AY. Ladd. 



On a Phenomenon of Metallic IttJIection. By Professor G. G. Stokes, F.E.S. 



The phenomenon which I am about to describe was observed by me many years 

 ago, and may not improbably have been seen by others ; but as I have never seen 

 any notice of it, and it is in some respects very remarkable, I think that a descrip- 

 tion of it will not be unacceptable. 



When Newton's rings are formed between a lens and a plate of metal, and are 

 viewed by light polarized perpendicularly to the plane of incidence, we know 

 that, as the angle of incidence is increased, the rings, which are at first dark- 

 centred, disappear on passing the polarizing angle of the glass, and then reappear 

 white-centred, in which state they remain up to a gi-azing incidence, when they can 

 no longer be followed. At a high incidence the first dark ring is much the most 

 conspicuous of the series. 



To follow the rings beyond the limit of total internal reflection we must employ 

 a prism. When the rings formed between glass and glass are viewed in this way, 

 we know that as the angle of incidence is increased the rings one by one open out, 

 uniting with bands of the same respective orders which are seen beneath the limit 

 of total internal reflection ; the limit or boundary between total and partial reflection 

 passes down beneath the point of contact, and the central dark spot is left isolated 

 m a bright field. 



Now when the rings are formed between a prism with a slightly convex base 

 and a plate of silver, and the angle of incidence is increased so as to pass the cri- 

 tical angle, if common light be used, in lieu of a simple spot we have a ring, which 

 becomes more conspicuous at a certain angle of incidence well beyond the critical 

 angle, after which it rapidly contracts and passes into a spot. 



As thus viewed the ring is, however, somewhat confused. To study the pheno- 

 menon in its purity we must employ polarized light, or, which is more convenient, 

 analyze the reflected light by means of a Nicol's prism. 



When viewed by light polarized in the plane of incidence, the rings show nothing 

 remarkable. They are naturally weaker than with glass, as the interfering streams 

 are so unequal in intensity. They are black-centred throughout, and, as with glass, 

 they open out one after another on approaching the limit of total reflection and 

 disappear, leaving the central spot isolated in the bright field beyond the limit. 

 The spot appears to be notably smaller than with glass under like conditions. 



With light polarized perpendicularly to the plane of incidence, the rings pass 

 from dark-centred to bright-centred on passing the polarizing angle of the glass, 

 and open out as they approach the limit of total reflection. The last dark ring to 

 disappear is not, however, the first, but the second. The first, corresponding in 

 order to the first bright ring within the polarizing angle of the glass, remains iso- 

 lated in the bright field, enclosing a relatively, though not absolutely, bright spot. 

 At the centre of the spot the glass and metal are in optical contact, and the reflection 

 takes place accordingly and is not total. The dark ring, too, is not absolutely 

 black. As the angle of internal incidence increases by a few degrees, the dark ring 

 undergoes a rapid and remarkable change. Its intensity increases till (in the case 

 of silver) the ring becomes sensibly black ; then it rapidly contracts, squeezing out, 

 as it were, the bright central spot, and forming itself a dark spot, larger than with 

 glass, isolated in the bright field. When at its best it is distinctly seen to bo 

 fringed with colour, blue outside, red inside (especially the former), showing that 

 the scale of the ring depends on the wave-length, being greater for the less refran- 

 gible colours. This rapid alteration taking place well beyond the critical angle is 

 verj' remarkable. Clearly there is a rapid change in the reflective properties of the 

 metal, wliich takes place, so to speak, in passing through a certain angle determined 

 bv a sine greater than unity. 

 " 187G. ^ 4 



