CH. X.] 



THE SPECTROSCOPE. 



243 



B 



C. The Spectroscopic Examinations of the Blood Pigments. 



The use of the Direct-vision Spectroscope. 



The instrument described is the small pocket spectroscope, with wave- 

 length scale attached, manufactured by Zeiss and Co. It is to be hoped that 

 an equally good instrument of home manufacture 

 will soon be forthcoming. The instrument (fig. 30) 

 consists of two tubes. The shorter tube A con- 

 tains a transparent photographic scale of wave- 

 lengths, with a mirror to project its image into the 

 field of vision. By means of the tube D this scale 

 can be focussed, and by the screw F it can be 

 adjusted to its proper position. The tube G con- 

 tains a series of alternating prisms of crown and 

 flint glass, arranged to allow the spectrum to be 

 observed by the eye in the line of the tube. The 

 tube B which slides on G has a vertical slit, the 

 width of which can be adjusted by turning the 

 collar E. 



To adjust the spectroscope: see that D and B 

 are pushed in as far as they will go. Look through 

 C towards the light with A to your left. Cautiously 

 turn E till the spectrum is just visible. (It is most 

 important to use an extremely narrow slit.) Slide 

 B out very slowly (in most instruments for 3^ 

 divisions as marked on the barrel G) till fine black 

 vertical lines can be seen in the spectrum, and 

 notice particularly a fine black line immediately to 

 the left of the narrow strip of yellow. This line is 

 known as the D line of Fraunhofer. The wave- 



C 



Fig. 30. Zeiss' direct- 

 vision spectroscope with 

 wave-length scale (x). 



length of it is -59 p, a position indicated on the scale by the division marking it 

 (the one to the right of 0-6) being produced further down than any other. If 

 necessary alter the position of the scale by turning the screw F until the D line 

 exactly coincides with the division mentioned. If the instrument has to be 

 adjusted at night-time, when the D line cannot be observed, set the scale 

 by use of the emission-spectrum of sodium (obtained by placing a few crystals 

 of common salt on the wick of a spirit lamp). The emission spectrum of 

 sodium exactly corresponds to the D line. The scale is so drawn that, if it be 

 set in position as described, the wave-length of light in any part of the visible 

 spectrum can be read directly. 



The numbers on the scale indicate wave-lengths in thousandths of a 

 millimetre, the unit being i p.. In the more recent patterns the wave-lengths 

 are given in millionths of a millimetre, the unit being i \. Thus the wave 

 length of the D line is 589 X. The other Fraunhofer lines that can be readily 

 observed with the instrument are C (657 X), E (527 X), b (518 X) and F( 4 86X) 



To observe absorption spectra : slightly open the slit of the spectroscope* 

 thus obtaining a better illumination. Direct the instrument to the light, and 

 place the test-tube containing the fluid to be examined directly in front of, 

 and touching, the tube B, with its axis parallel to the slit, taking care not to 

 interfere wth the illumination of the scale. With strong solutions of certain 

 pigments observed in this way it is often difficult to avoid illuminating the two 

 ends of the spectrum, the light being reflected from the sides of the tubes, and 

 not passing through the solution. To obviate this it is perhaps better to place 

 the solution in a beaker, remembering that the absorption of light increases 



