138 
ELEMENTARY CHEMICAL MICROSCOPY 
diameter has its lower end closed with a piece of plane glass 
cemented to it; it may be raised or lowered in its rubber sup¬ 
port (section of a rubber 
stopper) for the purpose 
of increasing or decreasing 
the thickness of the liquid 
layer which is being in¬ 
vestigated. 
The position of maxi¬ 
mum intensity of an ab¬ 
sorption band should 
always be determined by 
observing the situation of the vanishing point of the band after 
repeated dilutions. 
It should be borne in mind that the position of a band may 
be changed greatly through increased or diminished dissociation, 
and that the absorption bands given by a crystal may be quite 
different from those given by the same material in solution and 
furthermore that the absorption spectra are usually different in 
different directions through the crystal.^ 
Mechanical Stages. — In order to facilitate moving objects 
and to ensure certainty in covering a given area in quantitative 
work some form of device permitting accurate coordinate move¬ 
ments in the plane of the stage becomes essential. Such devices 
are known as mechanical stages and are indispensable in a great 
variety of microscopic work. Microscopes with a fixed mechani¬ 
cal stage are not desirable for ordinary chemical laboratory 
investigations, owing to the danger of spilling corrosive liquids. 
Attachable mechanical stages are far better for our purposes. 
These stages are of many forms though in principle and manner 
of employment all are similar. A type applicable to the chemical 
microscope (Fig. 25) is shown in Fig. 70. The large arm A 
encircles the pillar of the microscope and is held firmly in place by 
the set screw S, seating into a shallow slot made in the base of 
1 For the application of the spectroscope to determinative mineralogy see: 
Wherry, Microspectroscope in Mineralogy; Smithsonian.Misc. Coll. 66 (igi^), 
No. 5. 
"Observation Tube 
1 
Graduations 
>31 
Glass Cell containing 
Liquid 
Object Slide 
Fig. 69. Andrews’ Cell for Absorption 
Spectra. 
