Absorption of Light by the Earth's Atmosphere. 
7 
The foregoing investigation was undertaken for the purpose of obtain- 
ing a value of the coefficient of absorption true for South Africa. The 
results obtained by different astronomers at different parts of the earth's 
surface indicates that it is not the same for all regions. The presence of 
moisture, of dust particles, will modify to an appreciable extent the 
absorbing quality of the atmosphere. 
The present investigation is also, as the title indicates, simply pre- 
liminary. For a determination of the height of the atmosphere by this 
method the most refined photometric equipment is necessary. And I am 
confident that an investigation so carried out will yield results comparable 
in exactness and finality with any of the indirect ways of obtaining the 
height of the atmosphere. 
