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



illuminated. One can manage this by putting the object requiring the 

 greater amount of illumination on the stage of the microscope and light- 

 ing it with the Abbe illuminator. In lighting 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 petro- 

 leum lamp is excellent (§ 201-203). For objects with much blue or 

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

 acetylene light ought to be very satisfactory C§ 65). 



Furthermore, one should be on his guard against confusing the ordin- 

 ary 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. 



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

 material is of considerable bulk, a low objective (18 to 50 mm.) is to be 

 preferred. This depends on the nature of the object under examina- 

 tion, 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 entering the spec- 

 troscope 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 (§ 58, 102), and sometimes shade the object. 



§ 197. 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. When 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 exactly in position 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 too long it may be shortened by using 

 one of the mechanism screws on the side, or if that is not sufficient, 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. 



§ 198. Amount of Material Necessary for Absorption Spectra 

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

 give an absorption spectrum varies greatly with different substances, 

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

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



