242 The Rev. Prof. B. Powell an Apparatus for the 
in the way of its general introduction. Under this impression, 
derived in some measure from my own experience, it occurred 
to me to examine in what respects it might be capable of sim- 
plification. 
Following up this idea, after many trials I succeeded in con- 
triving an apparatus, which, at least for all general objects, 
answers the purpose, and is of extremely simple, easy, and 
cheap construction ; requiring in fact little more materials for 
its several parts than are either to be found already in the 
laboratory of every chemist, or may be readily procured, or 
constructed by the most ordinary workman. Of sucha contri- 
vance I gave a short account to the Chemical Section of the 
British Association at the Manchester Meeting, 1842, and a 
short description with a sketch representing the principle, is 
given in the reports of the Association for that year. 
It has appeared desirable however to offer, to those interested 
in the subject, some further details, as many may wish to be 
able to construct such an apparatus for themselves; and still 
more, as there are one or two improvements in thedetails which 
have suggested themselves since I gave the description just re- 
ferred to. I proceed therefore to describe more precisely the 
principle of the construction as well as its details. 
In the construction of M. Biot, the object viewed is the 
round disc of polarized light transmitted through the aperture 
of the diaphragm in the tube of the polarizer; in order to 
see this distinctly through a considerable thickness of liquid, 
it is indispensable that the liquid be contained in a tube 
having parallel ends of plate-glass. Such a tube of course 
requires to be constructed accurately ; the necessary supply of 
them of different lengths, and for comparative experiments, 
&c., is expensive; and there is a considerable difficulty in 
filling them, and fitting on the glass ends so as to exclude air- 
bubbles, &c. Again, the double-refracting prism, which is 
necessary to procure a separation of the two images, must be’ 
of the most accurate construction, so as to give images abso- 
lutely free from colour from refraction, in order to distinguish 
precisely the tints developed by the polarization. 
The main principle of my construction refers to both these 
sources of difficulty: with respect to the first, I employ com- 
mon test tubes without any other mode of termination than 
that furnished by the rounded bottom of the tube, however 
irregular, and the level surface of the liquid at the top, the 
tube being necessarily placed in a vertical position. Through 
such a tube however the image of the disc of polarized 
light will obviously be very irregular, even.if the liquid be 
perfectly transparent ; and no distinct image is seen if it be only 
