I78 CHAP. XXI. THE ENERGY OF THE SPECTRAL RAYS 



distinctness. When the vertical slit is widened the spectrum 

 is very bright, but no longer pure. The breadth of the slit 

 is so adjusted that there is a maximum intensity of light 

 without overlapping in the spectrum ; the Fraunhofer's lines 

 become indistinct when there is any overlapping. 



Instead of moving the plant-vessel from one region of 

 the spectrum to the other, which might cause mechanical 

 disturbance, it is preferable to slide the different spectral 

 rays on to the plant- vessel kept undisturbed in its own place. 

 The possibility of changing the position of the spectrum is 

 absolutely necessary for experiments with the Radiometer 

 for the determination of the energy of the different rays : 

 for the highly sensitive Radiometer has to be kept fixed 

 and protected from all disturbance. 



The sliding of the spectrum is effected by means of a 

 highly polished and perfectly plane mirror M, silvered on 

 the front. The stand of the mirror has a fine tangent screw 

 by which the different rays can be made to fall either on 

 the plant-vessel or on the sensitive strip of the Radiometer 

 placed near it. 



For the measurement of the energy of radiation, some 

 observers employ a linear thermopile, which is but moderately 

 sensitive. Greater sensitiveness was obtained by Langley 

 in his device of the Bolometer, where the change of resist- 

 ance in a wire caused by rise of temperature, due to the inci- 

 dent radiation, is measured by a Wheatstone Bridge. The 

 arrangement is somewhat complicated, and various pre- 

 cautions have to be taken to secure accurate results ; but 

 in Langley 's hands it has done much in advancing astro- 

 physical research. His values are, however, quite inapplic- 

 able at a different latitude and under different climatic 

 conditions. The energy of the different rays is modified 

 by the absorbing thickness of the strata of air through which 

 sunlight is being transmitted ; changes in the intensity are 

 thus introduced by meteorological conditions, the position 

 of the sun in the heavens, the time of the day, and so on. 

 It was therefore necessary to devise a special Radiometer 



