THE PURE SOLAR SPECTRUM 



177 



for the presence of infra-red rays to the left of the visible 

 spectrum. Though the dispersion is greater at the more 

 refrangible end, this is not a serious drawback in the quanti- 

 tative measurement by the method to be described for 

 determining the distribution of energy in the different 

 regions of the spectrum. 



The following are the details of the method by which a 

 pure spectrum was obtained. Sunlight is reflected by a self- 



VI B GYOR 



Fig. 48. Arrangement for obtaining a Pure Solar Spectrum 



c, cylindrical lens focusses line of light on slit s ; l, focussing 

 lens ; p, CS 2 prism ; M, reflecting mirror on a stand with 

 tangent screw (not shown) , by which the different rays may 

 be thrown in succession on the plant- vessel or on the sensitive 

 strip of the Radiometer. 



regulating heliostat on to a cylindrical lens c, which is fixed 

 in the closed window of the dark experimental room. This 

 gives a bright and intense line of light. A vertical and 

 adjustable slit s, placed at the focus, cuts off all but the 

 central rays. A photographic lens L forms a sharp image 

 of the slit at a distance of 1 metre. The carbon disulphide 

 prism is placed in the path of light and is adjusted at 

 the angle of minimum deviation (fig. 48). The projected 

 spectrum now shows the Fraunhofer's lines with great 



