240 SULPHUR BACTERIA 



prism (R), the collimator (C), and the prism (P), by which it is 

 refracted and passes into the telescope (T). It enters the lower 

 part of the slit at S and forms the upper spectrum. The light 

 from L2 passing through a glass screen at Ag is reflected by the 

 prism R and enters the upper part of the slit and forms the 

 lower spectrum. In comparison with the refracted light 

 the reflected light loses 8 per cent, of its intensity, which must 

 be allowed for in the calculation. In order to produce spectra 

 of the same intensity, the light Lj is kept fixed and the light 

 Li moved backwards or forwards until both spectra appear to 

 be precisely similar. 



nd 



ti 

 position^ HL 



" 'jst position 



Fig. 66. 



Let Ij and l^ be the respective intensities of L^ and L2. 

 Then if d^ and d^ be their respective distances when similar 

 spectra are produced, we have 



idy- [d^ ■ 100- • • • ^^> 



The coloured fluid is now placed in position (F), and 

 reduces the intensity of the lower spectrum. Next the light 

 Lj is moved backwards to some position at a distance D^ at 

 which the two spectra are once more of the same intensity. 



The insertion of the coloured fluid alters the intensity of 

 the light from Lg. Let its new value be Ig'. 

 . I2' 92 Ii 



• {d^Y ' 100 (D,)^ 



(2) 



