166 MICRO-SPECTROSCOPE AND POLAR/SCOPE [ CH. VI 



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 meth- 

 ods 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 general that for red or yellow objects, lamp-light gives very sat- 

 isfactory 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 h 

 therefore, not a source of possible confusion. 



§ 227. Objective 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 

 examination, however. In case of individual crystals one should 

 use sufficient magnification to make the real image of the crystal en- 

 tirely 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 entering the spectroscope with the light from the object 

 might obscure the absorption bands. For opaque objects illumin- 

 ating objectives are useful (Fig. 143, 144). 



In using high objectives with the micro- spectroscope one must 

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

 object. 



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

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

 ing 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 

 (§ 220). After an object is focused, it may be put exactly in posi- 

 tion 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 



