138 Prof. E. C. Pickering on the Polarization of 



clear, would introduce a large error into the results ; and care 

 was therefore taken to make these observations only on cloudy 

 days, 



The second method was to replace the slit of a large Babinet's 

 goniometer, or optical circle, by a NicoFs prism, which was free 

 to turn round its axis, the angle of rotation being measured by 

 a graduated circle and index. In the eyepiece of the observing- 

 telescope a NicoFs prism was placed, and in front of it quartz 

 wedges giving lines which were bright or dark-centred, accord- 

 ing as the transmitted ray was polarized vertically or horizon- 

 tally. On looking through the telescope, the field was seen to 

 be traversed by lines, which disappeared only when the Nicol 

 in the collimator was inclined 45° with the vertical. At any 

 other angle, «, the vertical and horizontal components were 

 cos 2 a and sin 2 a } and hence were equivalent to a beam polarized 

 vertically by an amount cos 2a. If now any object was inserted 

 between the two telescopes polarizing the light horizontally/?, 

 the bands would disappear only when"/?= cos 2a. Measuring 

 the four positions of disappearance and taking their mean, gave 

 an accurate measure of the polarization by a Table of natural 

 cosines, as with the polarimeter described above. Another way 

 of expressing the effect of this instrument is to say that the 

 bands disappear when the Nicol is so turned that the plane of 

 polarization shall be brought by the objectunder examination to an 

 angle of 45°. The method of measuring the polarization of the 

 reflected ray is now obvious. The pieces of glass are placed verti- 

 cally on the centre plate between the two telescopes, the latter set 

 at an angle of 2i, and the glass turned until the light is reflected 

 from its surface, so as to render the field bright. The Nicol is 

 then turned until the bands disappear, and its position recorded. 

 The angle between the telescopes is then altered so as to make i 

 successively 20°, 30°, 40°, &c, and the observation repeated. 

 Various adjustments must be made to eliminate constant errors, 

 but they need not be detailed here. One series was made with 

 a glass prism having an index of 1*52, a second with a plate of 

 coloured glass, a third with a sheet of plate glass, and others 

 with four and ten microscope slides. The latter were used 

 because the thickness of the plate glass was such that, when a 

 number of plates were placed between the telescopes, a portion 

 of the internal reflection would be lost. 



To measure the polarization by refraction, two similar methods 

 were employed. The plates were placed vertically over the centre 

 of a graduated circle, and a piece of ground glass was viewed 

 through them by the polarimeter. The plates were then set at 

 various angles, and the polarization measured in each case. All 

 these observations were made in cloudy weather, to eliminate the 



