SPECTROSCOPIC DETERMTNATTON OP JJTllIUM. 



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at the same height, iismg some object or part of the spectroscope as a 

 gauge. When nearl}" at the Umit of visibility of tJie hthiiim hue he 

 can make the Hne appear or not by varying the vertical position of the 

 loop in the flame less than a centimeter. The position of the loop in 

 the horizontal section of the flame also afl'ects the brilliance of the 

 lithium line, probably due in part to the distance from the slit. It 

 requires some practice and skiU, therefore, on the part of the assistant 

 holding the wu'e if concordant results are to be obtained. 



In fact, the success of the observations depends upon maintaining 

 strictly comparable conditions throughout an observation on both 

 standard and unknown solutions. One of the chief reasons for trying 

 the hj'drogen flame was to lessen the variations due to the uncertainty 

 of the position of the loop before the slit. With the conveniences at 

 hand for controUing the supply of hydrogen, however, there were 

 greater irregularities introduced by the varying size of the flame than 

 were due to the position of the loop in the flame. With the use of 

 illuminatmg gas it has been found that the pressure varies durmg the 

 day so as to cause some slight irregularity in the minimum amount of 

 lithium necessary to produce a spectrum and it is desirable to check 

 the standard frequently. The distance from the slit in the collimator 

 to the flame is a factor which may be kept constant, as may the width 

 of the slit. The folio wmg figures show something of the eft'ect of 

 the width of the slit on the sensitiveness of the test. These trials 

 were made with the hydrogen flame. 



Effect of the width of slit of collimator on the sensitiveness of the test. 



Up to about the width of 0.2 mm the lithium line is visible with 

 smaller amounts of lithium as the slit is made wider. If used wider 

 than 0.2 mm there is no very noticeable increase in brilliance in 

 the lithium line while if other salts are present the light from them 

 is so bright as to interfere seriously with the observation of the lith- 

 ium. As the whole method rests upon the fact of the observer seeing 

 or not seeing the lithium line, there enters also a question of the 

 personal equation. In all experiments it is possible to keep the 

 mind of the observer free from bias. No effect of the observer's 



