420 Cambridge Philosophical Society. 



formed, which are evidently of the nature of Newton's coloured 

 rings of thick plates, may be made to pass in a perfectly continuous 

 manner into the coloured bands observed by Dr. Whewell. 



The author has compared theory and experiment in various par- 

 ticulars, and has found the agreement perfect. It will be sufficient 

 to mention here one result of theory, which is of great generality 

 and of considerable elegance. It applies to the system of rings seen 

 by reflexion in a mirror, either plane or curved, when a luminous 

 point is placed anywhere near the axis, and the eye occupies any 

 other position likewise near the axis. The result is as follows :— 

 Join the eye with the luminous point, and likewise with its image, 

 whether it be real or virtual, and find the points in which the join- 

 ing lines, produced if necessary, cut the mirror. Describe a circle 

 having for diameter the line joining these two points. This circle 

 will be the middle line of the bright colourless fringe of the order 

 zero, and on each side of it the colours will be arranged in descend- 

 ing order. 



June 2. — On a new Elliptic Analyser. By Professor Stokes. 



After mentioning some of the inconveniences and inaccuracies 

 attending the use of a Fresnel's rhomb in the analysis of elliptically- 

 polarized light, and alluding to some other methods which had been 

 employed for the purpose, the author proceeded to describe a new 

 instrument which he had invented, and which he exhibited to the 

 meeting. In the construction of this instrument he had aimed at 

 being independent of the instrument- maker in all important points 

 except the graduation. The construction is as follows : — 



A brass rim or annulus is mounted so as to stand with its plane 

 vertical when ])laced on a table. Within this rim turns a brass gra- 

 duated disc ; and the angle through which it turns is read off by 

 means of verniers engraved on the face of the rim, and reading to 

 tenths of a degree. This disc is pierced at the centre, and carries 

 on the side turned towards the incident light a retarding plate of 

 selenite, of such a thickness as to give a difterence of retardation in 

 the oppositely polarized pencils amounting to about a quarter of an 

 undulation. In front it carries a liollow cylinder, turned on the 

 lathe along with the disc itself. Round this cylinder there turns a 

 collar containing a Nicol's prism, and carrying a pair of level-edged 

 verniers, by which the angle may be read oiF through which the 

 prism has been turned. Thus the retarding plate moves in azimuth 

 carrying the prism along with it, and the prism has likewise an in- 

 dependent motion in azimuth. 



In observing, the light is extinguished by a combination of the 

 two movements, in which case the elliptically-polarized light is con- 

 verted by the retarding plate into plane polarized, which is then ex- 

 tinguished by the Nicol's prism. On account of chromatic varia- 

 tions, the light is not, strictly speaking, extinguished, unless homo- 

 geneous light be employed, but only reduced to a minimum. There 

 are two principal positions of the retarding plate and Nicol's prism 

 in which the light is extinguished, or at least would be extinguished 

 if the incident light were homogeneous ; and for each principal 



