302 PROCEEDINGS OP THE AMERICAN ACADEMY 



distant mountain would be particularly valuable in this connection, as a 

 means of comparing the polarization of the sky with that of the light 

 received from the mountain. The relative intensity of the light from 

 the mountain and the adjacent sky should also be compared, since this 

 quantity would also vary with the clearness of the air. All of these 

 quantities are readily measured with this instrument. To compare the 

 light of the sky with that received from the mountain, it is only neces- 

 sary to conceal the sky line behind one of the bars, and thus compare 

 the relative intensities of the two rays. As the vertical component of 

 one ray will be compared with the horizontal component of the other 

 ray, the relative intensities will be modified by the amount of the polar- 

 ization. The observation should therefore be repeated, placing the sky 

 line of the mountain behind one of the adjacent bars. The polariza- 

 tion of the light received from the sky and the mountain should also be 

 determined, and we can then determine the relative intensities of the 

 four components. These measurements will be made more readily 

 if the polarimeter is inserted in the eyepiece of a telescope of low 

 power, placing the bars in the focus and the double-image prism be- 

 tween them and the field lens. The Nicol may be placed between the 

 field and eye lenses, or between the eye lens and the eye. 



