28 SPOTTISWOOBE. — POLARISATION OF LIGHT. 



(2.) The observation with a double-image prism is similar to 

 that with a Nicol. This instrument, as its name implies, gives the 

 images which would be seen through the Nicol in two rectangular 

 positions, both at once, so that they can be directly compared ; and 

 when in observing polarised light the instrument is turned so 

 that one image is at a maximum, the other is simultaneously at 

 a minimum. Both these methods of observation, (1) and (2), are 

 especially suitable for faint light ; because in such a case the eye 

 is better able to appreciate differences of intensity than differences 

 of colour. 



(3.) The observation with a biquartz differs from (1) only by 

 holding a biquartz (a right-handed and a left-handed quartz 

 cemented side by side) at a convenient distance beyond the Nicol, 

 and by observing whether colour is or is not produced. If the 

 Nicol be so turned that the two parts of the biquartz give the same 

 colour (choose the neutral tint, teint de passage, rather than the 

 yellow), we can detect a change in the position of the plane of 

 polarisation by a change in colour, one half verging towards red, 

 the other towards blue. This observation is obviously applicable 

 to a change in the plane, either at different parts of the pheno- 

 menon at the same time, or at the same parts at different times. 



(4.) We may use a Savart's polariscope, which shows a series 

 of coloured bauds in the field of view. For two positions at 

 right angles to one another, corresponding to the two critical 

 positions of a Nicol, these bands are most strongly developed ; 

 for two positions midway between the former the bands vanish. 

 In the instruments here furnished the plane of polarisation of 

 the observed light will be parallel to the bands when the central 

 one is light, perpendicular to them when the central band is dark. 



The instruments supplied will enable the officer charged with 

 tliem to repeat all the principal experiments in the polarisation of 

 light. After a little practice the observer will be able to make 

 his own selection of apparatus, and to apply the method of obser- 

 vation best suited to the particular circumstances in which he finds 

 himself placed. 



3. Insteuctions in the use of the Spectroscopes supplied 

 to the Arctic Expedition. By J. Norman Lockyer, 

 RII.S. 



The instruments supplied are as follows : — 



A. — An automatic 6-prism spectroscope of large dispersion 



for observations of the Sun. 

 B. — ^A direct-vision spectroscope by Merz for observations of 



the sun and of the aurora. 

 C. — A direct-vision spectroscope by Browning for observations 



of the aurora. 

 D. — A miniature spectroscope for observations of the aurora. 



