59 



MICRO-SPECTROSCOPE AND POLAR/SCOPE. 



The direct and the comparison spectrums should be about equally il- 

 • luminated. One can manage this by putting the object requiring the 

 greater amount of illumination on the stage of the microscope, and 

 lighting it with the Abbe illuminator. 



§ 141. Objectives to Use with the Micro-spectroscope. — If the 

 material is of considerable bulk a low objective (18 to 50 mm.) is to be 

 preferred. This depends on the nature of the object under examina- 

 tion, however. In case of individual crystals one should use sufficient 

 magnification to make the real image of the crystal entirely fill the 

 width of the slit. The length of the slit may then be regulated by the 

 screw on the side of the drum, and also by the comparison prism. If 

 the object does not fill the whole slit the white light entering the spec- 

 troscope with the light from the object might obscure the absorption, 

 bands. 



In using high objectives with the micro-spectroscope one must very 

 carefully regulate the light (§§ 39, 68), and sometimes shade the object. 



§ 142. Focusing the Objective. — For focusing the objective the 

 prism-tube is swung aside, and then the slit made wide by turn- 

 ing the adjusting screw at the side. When the slit is open, one can see 

 objects when the microscope is focused as with an ordinary ocular. 

 After an object is focused, it may be put exactly in position to fill the 

 slit of the spectroscope, then the knife edges are brought together till 

 the slit is of the right width ; if the slit is then too long it may be 

 shortened by using one of the mechanism screws on the side, or if that 

 is not sufficient, by bringing the comparison prism farther over the 

 field. If one now replaces the Amici prism and looks into the micro- 

 scope, the spectrum is liable to have longitudinal shimmering lines. 

 To get rid of these focus up or down a little so that the microscope 

 will be slightly out of focus. 



§ 143. Amount of Material Necessary for Absorption Spectra 

 and its Proper Manipulation. — The amount of material necessary to 

 give an absorption spectrum varies greatly with different substances, 

 and can be determined only by trial. If a transparent solid is under 

 investigation it is well to have it in the form of a wedge, then succes- 

 sive thicknesses can be brought under the microscope. If a liquid 

 substance is being examined, a watch glass with sloping sides forms an 

 excellent vessel to contain it, then successive thicknesses of the liquid 

 can be brought into the field as with the wedge shaped-solid. Fre- 

 quently only a very weak solution is obtainable, in this case it can be 

 placed in a homoeopathic vial or in some glass tubing sealed at the end, 

 then one can look lengthwise through the liquid and get the effect of a 

 more concentrated solution. For minute bodies like crystals or blood 

 corpuscles, one may proceed as described in the previous section. 



