April 27, 1871 | 
NATURE 
517 

case the components of the light oppositely polarised in the two 
sections are unequal, being as cos 223° to sin 221°; these com- 
ponents respectively fall 224° from the plane of reflection of the 
silver plate and from the perpendicular plane, and are each re- 
solved in the same proportion in these two planes. The weak 
component of the first, and the strong component of the second, 
are resolved into the normal plane, while the strong component 
of the first and the weak component of the second are resolved 
into the perpendicular plane. 
As bearing intimately on the subject of this paper, I will 
here quote a passage from a memoir presented by Fresnel to the 
French Academy of Sciences in 1817, and published, in abstract, 
in the ‘‘ Annales de Chimie,” t. xxviii., 1825 —- 
“If a thin crystallised plate be placed between two parallelo- | 
pipeds of glass crossed at right angles, in each of which the light 
previously polarised undergoes two total reflections at the incidence 
of 544°, first before its entrance into the plate (which we sup- 
pose perpendicular to the rays), and subsequently after its emer- 
gence ; and if, besides, the plate be turned so that its axis makes 
an angle of 45° with the two planes of double reflection, this 
system will present the optical properties of plates of rock crystal 
perpendicular to the axis, and of liquids which colour polarised 
light. When the principal section of the rhomboid with which 
the emergent light is analysed is turned round, the two images 
will gradually change colour, instead of experiencing only simple 
variations in the vividness of their tints, as occurs in the ordinary 
case of thin crystallised plates; besides, the nature of these 
colours depends only on the respective inclination of the primitive 
plane of polarisation and the principal section of the rhomboid, 
that is to say, of the two extreme planes of polarisation ; thus, 
when this angle remains constant, the system of the crystal'ised 
plate and the two parallelopipeds may be turned round the trans- 
mitted pencil without changing the colour of the images. It is 
this analogy between the optical properties of this little apparatus 
and those of plates of rock-crystal perpendicular to the axis 
which enabled M. Fresnel to foresee the peculiar characters of 
double refraction that rock-crystal exerts on rays parallel to the 
axis.” 
It does not appear that Fresnel, in any of his published me- 
moirs, has given any further modifications of this experiment, the 
importance of which has been almost entirely overlooked in ele- 
mentary treatises on light. He does not seem to have remarked 
that similar phenomena of successive polarisation are exhibited 
when the light incident on the crystallised plate is plane polar- 
ised, nor that the order of the succession of the colours depends 
on the position of the principal section with respect to the plane 
of polarisation, These circumstances are indeed necessarily in- 
cluded in the beautiful theory established by this eminent philo- 
sopher ; but I am not aware that they have hitherto been speci- 
fically deduced or experimentally shown. 
The apparatus (Fig. 1) affords also the means of obtain- 
ing large surfaces of uncoloured or coloured light in every state 
of polarisation, rectilinear, elliptical, or circular. 
It is for this purpose much more convenient than a Fresnel’s 
rhomb, with which but a very small field of view can be obtained. 
It must, however, be borne in mind that the circular and ellip- 
tical undulations are inverted in the two methods ; in the former 
case they undergo only a single, in the latter case a double re- 
flection. 
For the experiments which follow, the crystallised plate must 
be placed on the diaphragm E between the silver plate and the 
analyser, instead of as in the preceding experiments, between 
the polariser and the silver plate. : 
By means of a moving ring within the graduated circle D the 
silver plate is caused to turn round the reflected ray, so that while 
the plane of polarisation of the ray remains always in the plane of 
reflection of the glass plate, it may assume every azimuthal 
position with respect to the plane of reflection of the silver plate. 
The film to be examined and the analyser move consentaneously 
with the silver plate, while the polarising mirror remains fixed. 
In the normal position of the instrument the ray polarised by 
the mirror is reflected unaltered by the silver plate; but when 
the ring is turned to 45°, 135°, 225°, or 315", the plane of polari- 
sation of the ray falls 45° on one side of the plane of reflection 
of the silver plate, and the ray is resolved into two others polarised 
respectively in the plane of reflection and the perpendicular 
plane, one of which is retarded on the other by a quarter of an 
undulation, and consequently gives rise to a circular ray, which 
is right-handed or left-handed according to whether the ring is 
turned 45° and 225°, or 135° and 315. When the ring is 

turned so as to place the plane of polarisation in any, inter- 
mediate position be-tween those producing rectilinear and cir- 
cular light, elliptical light is obtained on account of the unequal 
resolution of the ray into its two rectangular components, 
_ Turning the ring of the graduated diaphragm from left to 
right when the crystallised film is between the silver plate and 
the analyser, occasions the same succession of colours for the 
same angular rotation as rotating the analyser from right to lett 
when the instrument is in its normal position, and the film is be- 
tween the polariser and the silver plate. ; 
To arrange the apparatus for the ordinary experiments of 
plane-polarised light without the intervention of the silver plate, 
all that is necessary is to remove the silver plate from the frame 
F, and to substitute for it a plate of black glass, which must be 
fixed at the proper polarising-angle. 
To convert it into a Norrenberg’s polariser, a silver mirror 
must be laid horizontal at H, and the instrument straightened, as 
shown at Fig. 3, so that a line perpendicular to the mirror shall 
correspond with the line of sight. The silver plate must be 
removed from the frame F, and a plate of transparent glass sub- 
stituted for it, which must be so inclined that the light falling 
upon it shall be reflected at the polarising-angle perpendicularly 
towards the horizontal mirror. The eye will receive the polarised 
ray reflected from the mirror, and the polarised ray will have 
passed, before it reaches the eye, twice through a crystallised 
plate placed between the mirror and the polariser. The result is 
the same as if, in the ordinary apparatus, the polarised plate had 
passed through a plate of double the thickness. 
_ Fig. 2 shows the addition to the apparatus when the coloured 
rings of crystals are to be examined by light circularly or 
elliptically polarised ; @ is the optical tube containing the lenses, 
which require no particular explanation, and 4 the condenser over 
which the plate is to be placed. 
C. WHEATSTONE 


SCIENTIFIC SERIALS 
In the Fournal of Botany for April, Mr. Hiern concludes his 
exhaustive paper On the Forms and Distribution over the World 
of the Batrachian Section of Ranunculus ; Mr. J. G. Baker his 
Monograph of the Liliaceous genus X7f/z07 ; and we have also 
the conclusion of the valuable List of New Species of Phane- 
rogamous plants published in Great Britain during the year 1870. 
Mr. Hiern’s paper concludes with a mathematical statement of 
the form of the leaf of water-plants dependent on the strength of 
the current. Mr. Carruthers reviews the Contributions to Fossil 
Botany published in Britain in 1870, which are very few in 
number. 
In the Scottish Naturalist for April, Dr. Lauder Lindsay 
commences his second paper on Natural Science Chairs in our 
Universities, and the editor, Dr. Buchanan White, concludes his 
details of ‘‘ Sugaring, how, when, and where to do it.”” Under the 
head of zoology, Dr. D. Sharp gives an interesting account of 
the Coleoptera of the Scotch Fir, and Mr. Robert Gray a history 
of the Capercaillie. The conrributions to phytology are a short 
paper on Scottish plant-names ; and a List of Mosses found in 
the vicinity of Forres, by the Rev. James Keith. Some of the 
shorter paragraphs contain also interesting information. We 
would suggest to the conductors of the WVatwralist whether 
it is not possible to avoid the very objectionable practice ot 
dividing their papers in the very middle of a sentence. The 
present number commences ‘‘. . . . waist.” in the midst 
of the Editor’s article on Sugaring, and concludes ‘* Knowledge 
islikelyto . . . .” in Dr. Lauder Lindsay’s on the Univer- 
sity Chairs. It is too great a stretch on the memory to expect 
an incomplete sentence to be kept in the head for three months ; 
and the previous number is not always at hand to remind one of 
the connection. 


SOCIETIES AND ACADEMIES 
LonDON 
Royal Society, April 20.—‘‘ Research on a New Group of 
Colloid Bodies, containing Mercury, and certain members of the 
series of Fatty Ketones.” By J. Emerson Reynolds, M.R.C.P. 
Edin., &c. 
** On the Existence and Formation of Salts of Nitrous Oxide,” 
By Edward Divers, M.D. 
