170 Sir David Brewster on the Polarization 



When the sky is clear the neutral point under the sun ap- 

 proaches to the sun as his altitude increases, and coincides with 

 the sun's centre when he is in the zenith. 



As the neutral points of Arago and Babinet may be seen 

 before the sun has risen, and after he has set, they are compa- 

 ratively distinct and limited in their area ; but as tne neutral 

 point below the sun never can be seen unless when the sun is 

 shining, it has a less denned boundary and a wider area, owing 

 to the flood of light in which it is generally enveloped. Hence 

 arises the great difficulty of seeing it, and of detecting the form 

 of the lines of equal polarization which surround it. For the 

 same reason, we can hardly expect to see the secondary neutral 

 point, which must accompany it, when it rises or sets on a sea 

 horizon in a condition to produce that phenomenon. 



On the Place of Maximum Polarization, and its Intensity. 



Next in importance to the determination of the place and 

 movements of the three neutral points is the determination of 

 the place and intensity of the maximum polarization of the 

 atmosphere. 



In order to obtain these elements, a polarimeter, or instru- 

 ment for measuring degrees of polarization, is required. M. 

 Arago constructed a very ingenious polarimeter, and I have de- 

 scribed twc forms of a polarimeter in the 'Transactions of the 

 Royal Irish Academy for 1841*; but these instruments are too 

 complex to be used from hour to hour during transient condi- 

 tions of the atmosphere, when observations must be made with 

 great facility and quickness. I was therefore obliged to use the 

 following instruments. 



1. Into one end of a tube 5 or 6 inches long and \\ inch 

 wide I inserted a band polariscope, and in the axis of the tube I 

 placed in a trough several (six to twelve) well annealed thin 

 glass plates with their surfaces inclined to the axis of the tube 

 at such an angle as to equal or compensate the average maximum 

 polarization to be measured. This compensation was effected 

 more simply by adding or removing one or more plates when 

 those in the trough had been previously placed at a fixed angle 

 to the axis of the tube. It is obvious that, by giving the pile 

 of plates a motion in one plane so as to vary the angle of refrac- 

 tion of the incident light, we should have an instrument for 

 measuring all degrees of polarization. I preferred, however, to 

 use a polarimeter all the parts of which were absolutely fixed. 



In looking through this instrument we have a circular field 



* Vol xix. part 2. 



