10 
patient after three years’ suffering from scrofulous disease of 
the hip-joint, and a large portion of the shaft of the tibia of 
another boy, whose leg was injured by a kick whilst playing at 
football. In both cases a most successful cure had been effected 
under the surgical care of his friend, Dr. Wm. Price, of Margate. 
A paper was read by Mr. Tos. Cusine, entitled, “ Notss 
ON THE PouaRizaTion OF LIGHT WITH A BRIEF ACCOUNT OF THE 
MaTHODS OF EMPLOYING IT IN CONNECTION WITH THE MICROSCOPE.” 
The author stated that he had prepared this paper, at the request 
of the President, in order to explain to the members of the Club 
a new arrangement of polarising apparatus, being one which had 
attracted considerable attention at the conversazione of the Croydon 
School of Art last month. Before proceeding, however, to describe 
the mechanism of this ingenious contrivance, Mr. Cushing thought 
it would be desirable to point out some of the various modes of 
polarising light in order to render the description of this apparatus 
more clear and intelligible. He then explained that the term 
polarisation was, at the suggestion of Sir Isaac Newton, given by 
the celebrated Huyghens to certain peculiarities possessed by rays 
of light which, having passed through crystals of Iceland Spar, or 
other double refracting media, acquired properties which were 
supposed to bear some analogy to those possessed by the opposite 
poles of a magnet. But whether this term is the best designation 
of the phenomena which is known as polarised light or not, it is 
clear that rays modified by the above and various other modes of 
treatment do present, as it were, sides on which they can be again 
reflected, and others on which they cannot. He then passed on to 
describe in general terms the method of polarisation by single 
refraction pointing out numerous substances, and the particular 
angles at which they polarise light, and stated his belief that even 
some metallic substances which are said to be incapable of 
polarising light have to some extent that power. The ordinary 
polarising apparatus supplied as an adjunct to the microscope was 
shown to consist of two Nicol’s prisms, not that two are necessary 
to polarise light—as this is done by a single prism of this con- 
struction—but because the second prism is necessary to ascertain 
that the light has been polarised in passing through the first. 
Hence the second is termed an analyzer. In the microscope the 
polarising prism is invariably fixed under the object while the 
analyzer is always placed above, but it has the choice of two 
positions, one just above the objective, and the other above the 
eye piece. Each position has its peculiar advantages and disadvan- 
tages, and must therefore be determined in each case according to 
the requirements of the observer. If on looking through the 
microscope, when the polarising apparatus is attached, the field 
should appear dark it will be found that by rotating one of the 
prisms through 90 degrees or a quarter of a revolution it will 
