﻿398 Royal Institution :— 



The line of motion of the moon's centre is obtained by ruling 

 through the mid points between A" and B", B" and C", C" and D". 

 The point on this line reached by the moon's centre at the moment 

 of greatest obscuration is found by bisection in M. Drawing through 

 M and S the bisecting line of greatest obscuration, the length of the 

 sagitta m s is determined. 



It is found by these observations that, 



The sun's diameter being taken at 530 



That of the moon is 540 



The length of the sagitta m s is 100 



These numbers, according to the proportions given in the Nautical 

 Almanac for the Radcliffe Observatory, would have been : — 



j Diameter of the sun 530*0 



Diameter of the moon 538*8 



Length of sagitta 99*1 



The agreement is quite close enough to justify the belief that, in 

 skilful hands, the method described may be in some cases very useful, 

 it being by no means limited to eclipses. It is so simple that one 

 can hardly suppose it not to have been already employed ; but I have 

 met with no notice of such being the case. 



During the progress of the eclipse three thermometers were ob- 

 served. One north of the house, screened from the sun and sky, 

 sank from 26° at ll h 40 m to 24°*4 at l h 25 m . One south of the 

 house, indirectly influenced by solar radiation on neighbouring ob- 

 jects, rose from 26°*75 at ll h to 27°'8 at noon, then sank to 26° at 

 12M0 m , and rose to 27°*3 at l h 35 ra . A third, on grass open to the 

 sky, sank from 27°*8 at 1 l h 40 m to 23°«5 at l h 25 m , and remained 

 at this point till l h 35 m . Though on a limited scale, the influence 

 both of solar and sky radiation is traceable in these observations. 



ROYAL INSTITUTION OF GREAT BRITAIN. 



Friday, February 3, 1871. — "On some Experiments on Succes- 

 sive Polarization of Light made by Sir C. Wheatstone." By W. 

 Spottiswoode, Esq., Treas. R.S. and R.I. 



The experiments which formed the subject of this discourse were 

 made by Sir Charles Wheatstone some years ago, but the pressure 

 of other avocations delayed their publication. The term " Succes- 

 sive polarization " was applied by Biot to denote the effects produced 

 when a ray of polarized light is transmitted through a plate of rock 

 crystal cut perpendicularly to the axis. The plane of polarization 

 is found to be changed on emergence and through a different angle 

 for each homogeneous ray. The introduction of instrumental means 

 for converting the plane polarization of the ordinary apparatus into 

 successive, or, as it is more commonly called, circular polarization, 

 and the explanation of the phenomena thence arising, constitute the 

 main purpose of the communication. 



Polarized light is distinguished from common light by the pre- 

 sence of certain peculiarities not ordinarily found ; but the peculi- 

 arities in question cannot be discerned by the unassisted eye, and 

 require special instrumental appliances for detection. A simple mode 



