144 MICRO-SPECTROSCOPE AND POLAR/SCOPE \CH. VI 



§ 204. Lighting for Micro-spectroscope. — For opaque objects 

 a strong light should be thrown on them either with a concave mirror 

 or a condensing lens. For transparent objects the amount of the sub- 

 stance and the depth of color must be considered. As a general rule 

 it is well to use plent3^ of light, as that from an Abbe illuminator with 

 a large opening in the diaphragm, or with the diaphragm entirely 

 open. For very small objects and thin. layers of liquids it may be 

 better to use less light. One must try both methods in a given case, 

 and learn by experience. 



The direct and the comparison spectra should be about equally 

 illuminated. 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. In lighting it is found in gen- 

 eral that for red or yellow objects, lamp- light gives very satisfactory 

 results. For the examination of blood and blood crystals the light 

 from a petroleum lamp is excellent. For objects with much blue or 

 violet, daylight or artificial light rich in blue light is best. 



Furthermore, one should be on his guard against confusing the 

 ordinary absorption bands with the Fraunhofer lines when daylight is 

 used. With lamp-light the Fraunhofer lines are absent and, therefore, 

 not a source of possible confusion. 



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

 the material is of considerable bulk, a low objective (16 to 50 mm.) is 

 to be preferred. This depends on the nature of the object under ex- 

 amination, 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 enter- 

 ing the spectroscope with the light from the object might obscure the 

 absorption bands. For opaque objects illuminating objectives are 

 useful (§28, 233.) 



In using high objectives with the micro-spectroscope one must 

 very carefully regulate the light ( Ch. II) and sometimes shade the 

 object. 



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

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

 adjusting 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 exactl} r in position to fill the slit of the 



