October 28,1871.] THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
313 
is not in use. The tubes are provided at both ends 
with screw-caps or end-pieces with glass centres (the 
caps at the lower ends being merely for convenience 
of cleaning), or the lower ends may be closed with 
circular plates of glass cemented on; and as it is 
essential to the proper transmission of light that tlio 
tubes shall be quite filled with liquid, the end-pieces 
are constructed as shown in fig. 2, where they aro 
marked a 2 a 2 , and where it will be observed that the 
glass centres are of considerable thickness. With 
this arrangement it results that any small quantity 
of air remaining in the tubes after they have been 
filled with liquid, must be displaced by the glass 
centres and forced to occupy the space round the 
bevelled edges of such glasses where its presence 
can cause no inconvenience. At the lower end of 
the tubes a small mirror <j (figs. 1 and 2) is hinged 
Fig. 4. 
to the lid of the box, and at the opposite end is at¬ 
tached in similar manner an optical arrangement 
consisting as shown in figs. 2 and 4 of two double 
prisms, and an eye-piece (the eye-piece and prisms 
being marked in fig. 2, h and It 2 respectively). 
So much in reference to the principal features of 
the apparatus which, as shown in fig. 3, appears 
when not in use as a box 20 inches by 5 inches by 
4^ inches, and is therefore portable and not liable to 
injury. Now for the mode of using it. 
We will, in the first instance, suppose that our 
standards, with which we wish to compare certain 
samples, are liquid; accordingly we open the lid of 
the box, adjusting it at an angle of about 45°, by means 
of the link and set-screw e f (figs. 1 and 3) and place 
the apparatus so that the mirror g may receive the 
light through a window direct from the sky. We 
then fill (either of the tubes with one of our 
standards, and the other tube with one of our 
samples; and applying an eye to the lens, we adjust 
the mirror so as to obtain the greatest amount of 
illumination. The rays of light reflected from the 
mirror take the course shown by the dotted lines in 
fig. 4, and we now observe in the lens a circular 
field divided down the centre by a perpendicular 
line. The tint of each semicircle corresponds to the 
colour of the liquid in the tube beneath it; and it is 
clear that we have here a means of comparing with 
great convenience and accuracy the two shades of 
colour. But, as has already been stated, the use of a 
liquid standard, at any rate in the case of petroleum, is 
open to objections; and Mr. Wilson has accordingly 
procured discs of stained glass about an inch in 
diameter, which when placed in the grooved holder 
i (figs. 1 and 2), the tube above being of course 
empty, produce exactly the same coloration of the 
corresponding side of the field, as if the tube were 
filled with the standard represented by the disc. 
The apparatus having been adopted by the Petroleum 
Association, discs have already been made, corre¬ 
sponding in colour to the standards of the Associa¬ 
tion, and petroleum standards of colour may now be 
carried in the pocket like so many halfpence. 
As remarked at the commencement, tins article 
refers particularly to petroleum, the desirability of 
