SPOTTISWOODE. — POLARISATION OF LIGHT. 27 



tions readily made, it appears quite worth while to repeat some of 

 them. 



The main features of polarisation in light from the sky are de- 

 scribed in the book which accompanies the instruments ; and they 

 may be observed with a Nicol's prism and biquartz, with a Savart's 

 polariscope, or even better with a Nicol, or a double-image prism, 

 alone. The statement that the polarisation is due to the scattering 

 of light from small globules of water suspended in the atmosphere 

 in the shape of mist must be regarded perhaps rather as a sugges- 

 tion than as an established fact ; and any observations made under 

 different atmospheric conditions capable of being brought to bear 

 on this question will be valuable. 



It is known that the light coming from a rainbow is polarised. 

 It will be worth while to examine whether the same is the case 

 with that from halos, &c. If this be so, observe the positions of 

 the Nicol, or double- image prism, in which the light is extinguished 

 (or most enfeebled) at different parts of the phenomenon. 



It has been suggested that the Aurora, inasmuch as it presents 

 a structural character, may afford traces of polarisation. Having 

 reference to the fact that the striae of the electric discharge in 

 vacuum tubes presents no such feature, the probability of the sug- 

 gestion may be doubted. But it will still be worth while to put 

 the question to an experimental test. 



If traces of polarisation be detected, it must not at once be 

 concluded that the light of the Aurora is polarised ; for the 

 Aurora may be seen on the background of a sky illuminated by 

 the moon, or by the sun, if not too far below the horizon, and 

 the light. from either of these sources is, in general, more or less 

 polarised ; therefore, if the light of the Aurora be suspected to 

 be polarised, the polariscope should be directed to an adjacent 

 portion of clear sky, free from Aurora, but illuminated by the 

 moon or sun as nearly as possible similar, and similarly situated, 

 to the former portion ; and the observer must then judge whether 

 the polarisation first observed be merely due to the illumination 

 of the sky. 



The light from the Ice blink should be also examined for traces 

 of polarisatiouc 



The presence of polarisation is to be determined : 



(1.) With a Nicol's prism, by observing the light through it, 

 by turning the prism round on its axis, and by examining 

 whether the light appears brightest in some positions and least 

 bright in others. If such be the case, the positions will be found 

 to be at right angles to one another. The direction of " the plane 

 " of polarisation " will be determined by that of the Nicol at 

 either of these critical positions. The plane of polarisation of 

 the light transmitted by a Nicol is parallel to the longer diagonal 

 of the face ; and, accordingly, the plane of polarisation, or partial 

 polarisation, of the observed light is parallel to the longer diameter 

 of the Nicol when the transmitted light is at its greatest intensity, 

 or to the shorter when it is at its least. 



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