138 



ELEMENTARY CHEMICAL MICROSCOPY 



Etr 



Observation Tnbe 



1 mnc "^ 

 Graduations 



S^/ Rubber 



Glass Cell containing 

 liquid 



Cement 



FIG. 69. 



Andrews' Cell for Absorption 

 Spectra. 



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. 1 



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, Microspectrpscope in Mineralogy; Smithsonian Misc. Coll. 65 (1015), 

 No. 5. 



