216 
ELEMENTARY CHEMICAL MICROSCOPY 
set of two exactly alike. Thick slices cut from one-hole rubber 
stoppers (or carefully bored compact corks) cemented to glass 
object slides serve to hold the tubes in a vertical position. The 
tubes are forced into the holes in the rubber or cork cells with a 
twisting motion and pushed down tightly until in contact with 
the glass slide. The apparatus should then be turned upside 
down and the contact between tube and glass slide examined 
with a hand magnifier to make sure of perfect contact and that 
no particles of the cell have been scraped off and lie between 
the ends of the tubes and the slide. Liquids for comparison 
may be introduced into the tubes by means of glass tubes drawn 
down fine enough to enter the colorimeter tubes. Air bubbles 
may be removed by means of a glass rod drawn down to the 
dimensions of a hair or by means of a platinum wire. 
It will be found desirable to blacken both the upper and the 
lower ends of the tubes and to wrap around the tubes black paper 
so as to avoid the entrance of side light. In the author’s labor¬ 
atory it is the custom to loosely coil around the tubes several 
thicknesses of dull black paper, glueing the last layer; when the 
glue has hardened a black paper tube results which may be 
slipped on the colorimeter tubes when observations are to be 
made and removed for cleaning the tubes. 
The method of procedure is the same as that followed when 
working with an ordinary colorimeter. A solution containing 
a known concentration of the colored substance is placed in 
one tube. The other tube contains the solution of the substance 
of unknown percentage composition. The tubes are so placed 
on the stages of the microscopes that their axes substantially 
coincide with the optic axes of the microscopes, the instruments 
having been previously focused and illuminated. Liquid is 
carefully removed from the tube which yields the darker field 
in the comparison eyepiece, until the colors of the halves of the 
field are of the same intensity. The depths of the columns of 
liquid are then determined (conveniently with a pair of dividers, 
a strong hand lens and a finely divided steel scale). The com¬ 
putations are made as usual.^ 
* Andrews’ Cells may also be advantageously used. See Fig. 69. 
