CH. VI.1 MICRO-SPECTROSCOPE AND POLARISCOPE. 131 



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. 



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

 the Aniici prism so that the spectrum will show the various spectral 

 colors going directly across it (§ 188-189) and carefully focus the slit. 

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

 ing as for the ocular micrometer (§ 161), 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 (§ 191). 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 § 195. 



§ 200. Absorption Spectrum of Permanganate of Potash. — Make 

 a solution of permanganate of potash in water of such a strength that 

 a stratum 3 or 4 mm. thick is transparent. Put this solution in a watch- 

 glass with sloping sides, and put it under the microscope. Use a 50 mm. 

 or 16 mm. objective, and use the full opening of the illuminator. Light 

 strongly. Look into the spectroscope and slowly move the watch-glass 

 into the field. Note carefully the appearance with the thin stratum of 

 liquid at the edge and then as it gradually thickens on moving the 

 watch-glass still farther along. Count the absorption bands and note 

 particularly the red and blue ends. Compare carefully with the com- 

 parison spectrum (Fig. 113). For strength of solution see § 202. 



§ 201. Absorption Spectrum of Blood. — Obtain blood from a 

 recently killed animal, or flame a needle, and after it is cool prick the 

 finger two or three times in a small area, then wind a handkerchief or a 

 rubber tube around the base of the finger, and squeeze the finger with 

 the other hand. Some blood will ooze out of the pricks. Rinse this off 

 in a watch-glass partly filled with water. Continue to add the blood 

 until the water is quite red. Place the watch-glass of diluted blood un- 



