CH. Vr\ MICRO-SPECTROSCOPE AND POLAR/SCOPE 145 



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 suffi- 

 cient, 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. 



§ 207. Amount of Material Necessary for Absorption Spectra 

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

 give an absorption spectrum varies great^ with different substances, 

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

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

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

 stance 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. 



MICRO-SPECTROSCOPE — EXPERIMENTS* 



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

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

 tral colors going directly across it (§ 19S-199) and carefully focus the 

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

 proceeding as for the ocular micrometer (§ 172), 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 (§ 198 ). With the lever move the com- 

 parison prism across half the field so that the two spectra shall be of 

 about equal width. For lighting, see § 204. 



*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 eye-piece. (Bleile, Trans. Amer. Micr. Soc. 

 1900, p. 8). 



