244 The Rev. Prof. B, Powell on Apparatus for the 
silvered mirror is placed beneath it, supported in such a man- 
ner as to be easily inclined, so as to throw the light in a proper 
direction. The great advantage gained is the saving of light, 
of which of course much is lost in the reflexion at the glass: 
and as a considerable number of the solutions we wish to ex- 
amine are but imperfectly transparent, this saving becomes 
important. 
In order to determine the direction of rotation, or the right 
or left-handedness of the polarization, it is necessary to com- 
pare two different thicknesses of the liquid: in other in- 
stances it is interesting to compare the effects of two different 
liquids: for these purposes it is extremely convenient to have 
a contrivance by which two tubes can be brought in succession 
into the apparatus without deranging the adjustment of the 
other parts. 
For different fluids differentlengths of tube are required; 
for some highly energetic, as oil of turpentine, with a length 
of 5 or 6 inches tints are seen; with most, solutions of sugar, 
&c., from 12 to 18 or even 24 inches are necessary. Hence 
the apparatus should be so contrived as to admit of the eye- 
piece being slid up and down as occasion requires. 
All these conditions are fulfilled in the construction of which 
T annex a sketch for the convenience of those who may wish to 
construct similar ones; and of which the following few details 
will abundantly suffice for explanation. 
Fig. 1. gives a general perspective view of the whole in 
its latest form. Fig. 2. is the lower part according to the first 
construction. (P) is the polarizing part, (A) the analysing. 
In each of these figures (s) is the silvered mirror which 
first receives the ray (either from a flame or the sky), which 
is thence thrown on the polarizer (p), which in fig. 1 isa 
Nicol prism fitted into the hole (¢) in fig. 2, a plain glass re- 
flector inclined 353° to the axis, the mirror (s) being ca- 
pable of inclination to bring the ray into the proper direction. 
In fig. 1. the course of the ray is represented by the dotted 
line, when no medium is interposed, from its first reflexion, 
through the analyser, to the eye (e). A section of the analyser 
is given in fig. 3, in which (7) is the rhomb, (2) the lens, and 
(&) the small hole in the bottom ofthe short tube or box con- 
taining the rhomb : (m) is the section of the circular rim with 
verniers at the openings (v, v), through which the graduation 
on the circle (n) beneath is read. ‘The circle (m) is moved 
round with the tube by means of its milled edge: (7) is fixed 
to the stand. The arm of the stand (H) which carries (A) 
should be capable of moving up and down according to the 
height of the tubes employed, and of being fixed by a clamping 
