232 



SIR DAVID BREWSTER 



On the Place of Maximum Polarisation, 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 maxi- 

 mum polarisation of the atmosphere. 



In order to obtain these elements, a polarimeter, or instrument for measuring 

 degrees of polarisation, is required. M. Arago constructed a very ingenious 

 polarimeter, and I have described two 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 conditions 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 G inches long and 1^ of an 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 

 polarisation to be measured. This compensation was effected more simply b}^ 

 adding or removing one or more plates when those in the trough had been pre- 

 viously 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 refraction 

 of the incident light, we should have an instrument for measuring all degrees of 

 polarisation. 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 SA, and when we 

 direct it to the region of maximum polarisation, with 

 the polarised bands parallel to SA, S being towards the 

 sun, we shall see an interruption in the bands somewhere 

 between S and A. If this interruption, or point of com- 

 pensation, is at the point 2 in SA, I call S2 the measure 

 of the maximum degree of polarisation at the time of 

 observation. After some practice, I had no difficulty in 

 estimating by the eye when the neutral line was at 1 

 or 1^, or 2 or 2^, without placing marks at I, 2, 3, &c., 

 on a plate of glass at the end of the tube. 



Having found that So Si S2, &c., corresponded with degrees of polarisation 

 measured by the rotation of the plane of polarisation, I thus had a measure of 

 the maximum polarisation of the atmosphere at the time of observation. f 



When it was necessary to measure very small degrees of polarisation, I pre 

 ferred using a polarimeter with a single plate, to one with a pile of plates 



Fig. 4. 



* Vol. xix. part ii. 



t See Phil. Trans. 1830, pp. 69, 133, 145, 287, and Trans. Irisli Acad., vol. xix. part ii. 



