96 



METHODS OF OBSERVING ABSORPTION SPECTRA. [BOOK I. 



difficulty in drawing up a map which shall represent the relative and 

 absolute position of any lines or bands observed in a given spectrum. 



The absorption bands which form the characteristic features in the 

 spectra of blood and certain other animal liquids do not admit of having 

 their limits determined with the same sharpness and precision as is possible 

 in the case of the bright lines in the spectra of incandescent metals or in 

 that of the lines of Frauenhofer in the solar spectrum. It would therefore 

 be mere pedantry to express their position or extent on a wave-length scale 

 to one ten-millionth of a millimetre. In this work all drawings of 

 spectra will be accompanied by a scale of wave-lengths, and the position 

 and extent of bands usually expressed in millionths of a millimetre. 



Micro- spec- Where very small quantities of a solution are to be 



troscopes. examined these may be introduced into small cells made by 



cementing sections of barometer tubing of various lengths and diameters 

 to glass slides. Such a cell may be made which only requires two or 

 three drops of fluid in order to fill it. Instead of employing an ordinary 

 spectroscope we may in this case with advantage employ some form of 

 micro-spectroscope. 



FIG. 20. 



ZEISS'S MJCRO-SPECTROSCOPE. 



The instrument consists of a drum A (Fig. 20) interposed between the field-lens and 

 eye-lens of an eye-piece. Within the drum there is a slit which by means of screws 

 H and F (Fig. 21) can be lengthened or shortened and made wider or narrower ; 

 it also contains a prism wherehy light coming from an aperture in a stage at the 

 side of the drum is totally reflected in the direction of the optic-axis of the eye- 

 piece. Over the eye-lens of the eye-piece is situated the combination of prisms 

 with the measuring apparatus ; this, which is the spectroscope proper, revolves 

 around the eccentric K (Fig. 20) : it can either be moved away from the eye-lens or 

 brought over it, and retained there by the catch L. At N is placed the scale of 

 wave-lengths (see Fig. 20), which is illuminated by the mirror 0. The screw P 

 and the spring Q are employed to alter the relation of the scale to the spectrum. 

 The former is always set by the observer so that Frauenhofer 's line D corresponds 

 to division 58-9. 



