14 REPORT — 1851. 



upon which the silver was polished, using fresh rouge and ammonia soap just as if 

 it was speculum metal. The spirit of turpentine had long been exposed to the light, 

 and consequently was slightly adhesive. Silver was several times polished on this 

 surface successfully, and a plain silver speculum so polished performed well, giving 

 perfectly sharp images. I have been thus minute in explaining what I believe to be 

 the rationale of the process as a guide to others, because, having as yet practised it 

 but little, I may perhaps have omitted to notice some things apparently non-essen- 

 tial, but which are really not so. 



On a new Elliptic Analyser. By Prof. G. G. Stokes, M.A.., F.R.S. 



After alluding to various methods which had been employed in investigating experi- 

 mentally the nature of elliptically-polarized light, that is to say, the elements of the 

 ellipse described, the author exhibited and described a new instrument which he had 

 invented for the purpose. In its construction he had aimed at being in all import- 

 ant points independent of the instrument- maker, assuming nothing but the accuracy 

 of the graduation. 



The construction is as follows : — A brass rim, or thick annulus, is fixed on a 

 stand, so as to have its plane vertical. A brass circle, graduated to degrees, turns 

 round within the annulus, and the angle through which it is turned is read by ver- 

 niers engraved on the face of the annulus. The brass circle is pierced at its centre, 

 and carries on the side turned towards the incident light a plate of selenite, of such 

 a thickness as to produce a difference of retardation in the oppositely polarized 

 pencils amounting to about a quarter of an undulation for rays of mean refrangibi- 

 lity. On the side next the eye the brass circle carries a projecting collar, and round 

 this collar there turns a moveable collar carrying verniers, and destined to receive a 

 Nicol's prism. 



The observation consists in extinguishing the light by a combination of the two 

 movements. The retarding plate converts the elliptically-polarized light which has 

 to be examined into plane-polarized, and this plane-polarized light is extinguished 

 by the Nicol's prism. There are two distinct positions of the retarding plate and the 

 Nicol's prism in which this takes place. In each of these principal positions the 

 retarding plate and the Nicol's prism may be reversed (i. e. turned through 180°), 

 and the means of the readings in these four subsidiary positions may be taken for 

 greater accuracy. The readings of the lixed and moveable verniers in each of the 

 two principal positions are four quantities given by observation, which determine 

 four unknown quantities, namely, (1) the index error of the fixed verniers, or, which 

 comes to the same, the azimuth of the major axis of the ellipse described by the 

 particles of sether, measured from a plane fixed in the graduated circle ; (2) the 

 ratio of the axes of the ellipse ; (3) the index error of the moveable verniers ; (4) 

 the retardation due to the retarding plate. The unknown are determined by the 

 known quantities by certain simple formula given by the author. 



Let these unknown quantities be denoted by I, tan zs, i, and g, respectivelj% 

 the latter being reckoned as an angle, at the rate of 360^ to an undulation. Let 

 R, r be the readings of the fixed and moveable verniers respectively in one of the 

 principal positions, R', »•' the corresponding readings in the other ; then 



1_ R'+R . ,•_»•'+'• . 



2*2' 



sin (r'—r) tan (j-'— >•) 



cos 2a;=^ — -^, — =^; cos p=-^ y^, — ^• 



sm (R'—R) ^ tan (R'—R) 



The author stated that he had made a good many observations with this instru- 

 ment for the sake of testing its performance, and that he had found it very satisfac- 

 tory. Inasmuch as light is not homogeneous, the illumination never vanishes, but 

 only passes through a minimum, and in passing through the minimum the tint 

 changes rapidly. This change of tint is at first somewhat perplexing ; but after a 

 little practice, the observer is able to point mainly by intensity, taking notice of the 

 tint as an additional check against errors of observation. The accuracy of the ob- 

 servations is a little increased by the use of certain rather pale-coloured glasses. 



