1871.] On a Decennial Variation of Temperature at the Cape. 389 



45° and 225° or 135° and 315°. When the ring is turned so as to place 

 the plane of polarization in any intermediate position between those pro- 

 ducing rectilinear and circular light, elliptical light is obtained, on account 

 of the unequal resolution of the ray into its two rectangular components. 



Turning the ring of the graduated diaphragm from left to right when 

 the crystallized film is between the silver plate and the analyzer, occasions 

 the same succession of colours for the same angular rotation as rotating 

 the analyzer from right to left when the instrument is in its normal posi- 

 tion and the film is between the polarizer and the silver plate. 



X. 



To arrange the apparatus for the ordinary experiments of plane -polarized 

 light without the intervention of the silver plate, all that is necessary is to 

 remove the silver plate from the frame F, and to substitute for it a plate 

 of black glass, which must be fixed at the proper polarizing-angle. 



To convert it into a Norrenberg's polarizer, a silvered mirror must be laid 

 horizontal at H, and the instrument straightened, as shown at fig. 3, so 

 that a line perpendicular to the mirror shall correspond with the line of 

 sight. The silver plate must be removed from the frame F, and a plate of 

 transparent glass substituted for it, which must be so inclined that the. 

 light falling upon it shall be reflected at the polarizing-angle perpendicu- 

 larly toward the horizontal mirror. The eye will receive the polarized 

 ray reflected from the mirror ; and the polarized ray will have passed, 

 before it reaches the eye, twice through a crystallized plate placed between 

 the mirror and the polarizer. The result is the same as if, in the ordinary 

 apparatus, the polarized ray had passed through a plate of double the 

 thickness. 



Fig. 2 shows the addition to the apparatus when the coloured rings of 

 crystals are to be examined by light circularly or elliptically polarized : 

 a is the optical tube containing the lenses (which require no particular 

 explanation), and b the condenser, over which the plate is to be placed. I 



II. "On an approximately Decennial Variation of the Temperature 

 at the Observatory at the Cape of Good Hope between the years 

 1841 and 1870, viewed in connexion with the Variation of the 

 Solar Spots." By E. J. Stone, F.R.S., Astronomer Royal 

 at the Cape of Good Hope. In a Letter to the President. Re- 

 ceived February 21, 1871. 



Royal Observatory, Cape of Grood Hope, Jan. 17, 1871. 

 Dear Sir, — I enclose a curve of the variation of the annual mean tem- 

 perature at the Cape deduced from observations extending from 1841 to 

 1870 inclusive. I have carefully examined the zero-points of all the ther- 



