7 2 
FIRST REPORT— 1881 , 
the instrument, with a variety of finely executed elliptical 
mirrors, &c. In the reflecting microscope of Sir Isaac Newton, 
the object is placed directly in the focus of the speculum, and 
the image is formed in that of the eye-glass; and thus, having 
only one additional surface in the essential parts of the instru¬ 
ment, it must be considered as next in simplicity to the single¬ 
lens microscope. 
In this construction the object must be placed in the axis of 
the tube, where it is difficult to provide sufficient illumination, 
and it is this defect which the new construction is intended to 
obviate. A large hole is cut in the tube between the object 
and the speculum, to allow the light to fall upon the former 
when it requires to be viewed as an opake object, and all the 
lower parts of the tube are lined with black velvet to absorb 
the irregular light. A large lens is also occasionally employed 
to concentrate the light. Transparent objects require a small 
oval mirror to be placed immediately behind them ; this mirror 
receiving a concentrated light from a lens, fixed in a sliding 
piece on the side of the tube, reflects it through the object to 
the speculum. The objects, to be placed in the centre of the 
tube, are attached to thin brass wires in wooden handles, and 
kept separately in a box. 
This construction of the reflecting microscope has a great 
advantage in point of distinctness, from there being only one 
necessary reflection between the object and the image, and will 
be found particularly suitable for the examination of opake 
objects, on account of the large aperture of the speculum, 
compared to its focal length.—It is therefore recommend¬ 
ed as an excellent working tool to the scientific inquirer, 
who will disregard the little trouble required in its manage¬ 
ment. 
The Secretary then read a descripton by Dr. Brewster of an 
Instrument for distinguishing Precious Stones and Minerals. 
The object of this instrument is to distinguish mineral bodies 
by the relative quantity and colour of the light reflected from 
their surfaces, when placed in contact with fluids of different 
refractive powers. The surfaces employed for this purpose 
may be either natural or artificial, so that the method is equally 
applicable to regular crystals, and to gems cut into artificial 
forms. If a fluid, of a given refractive and dispersive power, 
is placed on the surface of a mineral of the very same refrac¬ 
tive and dispersive power, there will be no light whatever re¬ 
flected from their separating surface ; but in proportion as the 
fluid and the solid differ in these respects, in the same pro¬ 
portion will the quantities of light differ which are reflected at 
