intensity of the light which would fall into the slit of the 
spectroscope if the Nicol’s were removed. 
A plane parallel piece of glass, acting as mirror, is fixed 
unto the small table, the centre of which coincides with the 
centre of the large goniometer table. The parallel sides can 
be adjusted by means of 3 screws until they are vertical. 
This mirror reaches to such a height that the horizontal 
plane laid through the top of the plate would bisect the 
tube containing the two Nicols. 
The light which is to be examined falls through a tube 
containing one Nicol, and is reflected by means of the 
plane parallel mirror into the lower half of the spectroscope. 
If the ray of light is reflected at the angle of polarization 
the intensity of this light can be reduced to nothing by 
means of the rotation of the Nicol. 
On placing the standard light in front of the tube con- 
taining the two Nicols and allowing the light which is to 
be examined to be reflected into the spectroscope on the 
mirror through the tube containing one Nicol, the mirror 
being placed at the angle of polarization, we observe in the 
spectroscope the two spectra one above the other, and by 
turning the Nicols we can reduce the intensity of the 
brighter light to that of the weaker for any colour we like. 
The positions of the Nicols will enable us to find the 
relative intensity of the two lights for the different colours. 
MICROSCOPICAL AND NATURAL HISTORY SECTION. 
December 6th, 18*75. 
Charles Bailey, Esq., Vice-President of the Section, in 
the Chair. 
Mr. SideboT’HAM, F.R.A.S., sent for exhibition some sand 
from a river farmland of New Guinea, containing particles 
of gold, magnetic and n on-magnetic iron, foraminiferse, 
silicified fragments of echini, and shells. 
