258 USING THE MICRO-SPECTROSCOPE [Ch. VIII 



§ 409. Focusing the objective. — For focusing the objective the 

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

 adjustable screw at the side. If 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 

 microscope, 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. 



§ 410. Amount of material necessary for absorption spectra and 

 its proper manipulation. — The amount of material necessary to 

 give an absorption spectrum varies greatly w T ith 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. 

 Frequently 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. See also § 420. 



Micro-spectroscope Experiments 



§ 411. Put the micro-spectroscope in position, arrange the slit 

 and the Amici prism so that the spectrum will show the various 

 spectral colors going directly across it (§ 402), and focus the slit. This 

 may be done either by swinging the prism-tube aside and proceeding 

 as for the ocular micrometer (§ 240), or by moving the eye lens of 

 the ocular up and down while looking into the micro-spectroscope until 



