USEFUL MICROSCOPE ACCESSORIES 113 



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 in- 

 vestigations, owing to the danger of spilling corrosive liquids. 



FIG. 57. Attachable Mechanical Stage. 



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. 3) is shown in Fig. 57. The large arm A en- 

 circles 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 the 

 pillar. This ensures replacing the stage each time in exactly 



