Ch. VIII] USING THE MICRO-SPECTROSCOPE 257 



thousandth mm., or 0.5892/^. The last would be indicated thus XD 

 = 0.5892M. 



§ 407. Lighting for the micro-spectroscope. — For opaque objects 

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

 or condensing lens. For transparent objects, the amount of the 

 substance and the depth of the color must be considered. As a 

 general rule it is well to use plenty of light, as that from a substage 

 condenser with a large opening in the diaphragm or with the dia- 

 phragm 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. 

 In hghting it is found in general 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 daylight is 

 best (§ 92). 



Furthermore, one should be on his guard against confusing the 

 ordinary absorption bands with the Fraunhofer lines when daylight 

 is used. With lamplight the Fraunhofer lines are absent and, there- 

 fore, not a source of possible confusion. 



§ 408. 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 

 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 com- 

 parison 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. 



In using high objectives with the micro-spectroscope one must 

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

 object. 



