6 REPORT—1846. 
view of more readily gaining for them the notice of those interested in optics. This 
peculiar action, it will be remembered, is of this kind: first, that the metals (and 
metallic sulphurets, &c.) have no angle of complete polarization for common light ; 
and secondly, that a plane polarized ray becomes elliptically polarized after reflexion 
from their surfaces, whereas it remains plane polarized after reflexion from glass and 
such like bodies. Endeavours have naturally been made to account for these phzeno- 
mena on the principles of the undulatory theory ; and always, apparently, on the suppo- 
sition that the laws of reflexion from transparent (uncrystallized) bodies were already 
rigorously given by Fresnel’s formulz, but that a new and distinct theory was required 
for metallic reflexion: thus assuming that the two classes of phenomena were ab- 
ruptly separated, without any intermediate links of connexion. It has, indeed, long 
been known that several transparent or translucent substances have no angle of com- 
plete polarization. Thus Biot (Traité de Physique, iv. 288) has excepted sulphur 
and the diamond; and Sir John Herschel (Optics, Art. 845 ; see also 831) excludes 
from the general rule, besides the metals, those substances which have the adamantine 
lustre; which term is applied, in Mohs’s system of crystallography, to several of the 
minerals to be presently spoken of, as resembling the metals in another respect. I 
do not know that any writer, except Mr. Green (in Camb. Phil. Trans. vol. vi.), has 
stated this exemption to be general for all substances having a high refractive index; 
but it is important to recall this experimental fact to our attention, on account of its 
coincidence and harmony with the new result which I have now to state. It consists 
in this: that these same highly refractive substances resemble the metals also in a 
second respect—that they confer elliptic polarization on a plane polarized ray reflected 
from them. The following list of substances, in which this property was observed, 
will be found to contain most of those at the top of Sir D. Brewster's list of refractive 
indices :-— 
Indigo—which is remarkable for possessing the metallic lustre without con- 
taining any metal. 
Artificial realgar. 
Diamond—of which three specimens were tried. 
Sulphuret of zine in transparent crystals. 
Glass of antimony—translucent. 
Sulphur—melted on a polished slip of zine foil. 
Tungstate of lime—transparent. 
Carbonate of lead in crystals, clear and limpid as glass. 
Hyacinth, or zincon—translucent. 
Arsenious acid. 
Garnet. 
Idocrase. 
Helvine. 
Labrador hornblend. 
Of which the last five possess the property in a very slight degree only. The test 
used in every case was the dislocation of the rings of a plate of cale spar; of which a 
very good specimen was used, capable of exhibiting eight or nine red rings: and all 
the experiments were made by candle-light, which is indispensable, It will secure 
greater confidence in these results to say, that all the specimens which I submitted to 
Prof. Powell’s examination, in a different instrument, were found by him to produce 
the above effect; and from his published observations several more cases may be 
quoted in confirmation of the general result: such are—chromate of lead, litharge, 
plumbago, and Indian ink. The natural conclusion from these facts appears to be, 
that in a perfect mathematical theory of reflexion, both cases should be embraced in 
one set of formule, of which some terms or coefficients should be insensibly small, 
except when the refractive index was very large ; that, strictly speaking, no substances 
completely polarize common light at any angle, but that the residue of unaltered light 
is too feeble to affect the eye, when the refractive index is below a certain limit; 
and that plane polarized light always becomes elliptically polarized, but that the vir- 
tual difference of paths of the two compact vibrations parallel and perpendicular to 
plane reflexions is insensibly small, except the refractive index surpass a certain value 
greater than the refractive indices of felspar and sapphire, which I found to produce 
no dislocation of the rings. It is remarkable, that such formule have some time 
