CH. VI ] MICRO-SPECTROSCOPE AND POLAR/SCOPE 167 



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 longi- 

 tudinal shimmering lines. To get rid of these focus up or down a 

 little so that the microscope will be slightly out of focus. 



§ 229. Amount of Material Necessary for Absorption 

 Spectra and its Proper Manipulation. — The amount of material 

 necessary to give an absorption spectrum varies greatly with differ- 

 ent substances, and can be determined only by trial. If a transpar- 

 ent solid is under investigation it is well to have it in the form of a 

 wedge, then successive thicknesses can be brought under the micro- 

 scope. 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 ob- 

 tainable ; 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. 



MICRO-SPECTROSCOPE — EXPERIMENTS* 



§ 230. 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 (§ 220, 221) and focus the 

 slit. This may be done either by swinging the prism- tube aside 

 and proceeding as for the ocular micrometer (§ 188), or by moving 

 the eye-lens of the ocular up and down while looking into the micro- 

 spectroscope until the dark lines of the solar spectrum are distinct. 

 If they cannot be made distinct By focusing the slit, then the light 

 is too feeble or the slit is too wide (§ 220). With the lever move 

 the comparison prism across half the field so that the two spectra 

 shall be of about equal width. For lighting, see § 226. 



*If one does not possess a micro-spectroscope, quite satisfactory results may 

 be obtained by using a microscope with a 16 to 12 mm. objective and a pocket, 

 direct vision spectroscope in place of the eyepiece. (Bleile, Trans. Amer. 

 Micr. Soc. 1900, p. 8). 



