112 



MICROSCOPIC TECHNIQUES 



windows, removable for cleaning. An aspirator tube is fitted into 

 the top of the box to remove vapors and the box itself is placed on 

 alignment pegs to maintain fixed optical relations. The disposition 

 of the spark gap relative to the spectrograph is shown in Figure 8. 

 As mentioned previously, the horizontal slit in the side of the box 

 facing the spectrograph serves to screen out electrode lines from 

 the tissue spectra and to obviate the need of cleaning the electrodes 

 oftener than once a month. The end of a 2 in. length of Pyrex tubing 

 (2-3 mm. inside diameter) or rod (2-3 mm. diameter) is centered 

 in the spark gap. It has been shown that the glass does not affect 

 the spectra. Bits of cellular material (3-6 [A.) are placed on the 

 end of the glass so that they will be in the center of the spark. A 



To spark generator 



Small Pyrex tube carrying bit of tissue 

 l" 



1.4 cm. between faces 



Spectrograph collimator 



14.0 cm. *^ 



\ *.> \ < — 5.3 cm. — »4< 



1.3 cm.-^ 



Fig. 8. Diagram of disposition of the spark gap 



relative to the spectrograph. 



From Williams and Scott (1935) 



film of purified Eastman gelatin may be helpful in effecting the 

 adherence of dry material. A strip of tissue, 5 mm. or more long, 

 may be placed in the glass tube and fed into the spark with a push 

 rod; and in a similar manner liquid taken up on a small strip of 

 ashless filter pulp can be subjected to test. In the latter case, con- 

 trol spectra for the pulp alone must be obtained. 



A Gaertner L250W quartz prism spectrograph taking 3i/4 X 4l^ 

 in. photographic plates was used. The spectrograph slit was fixed at 

 0.05 mm., and Eastman "50" plates were employed for high sensi- 

 tivity and Eastman Process plates for high contrast. Thirty ex- 

 posures may be made on each plate. Two microscopes with a com- 

 parison ocular were utilized for comparing the positions and inten- 

 sities of lines on different plates. 



Faint iron lines from the electrodes are apparent in the spectra 



