18 Prof. A. W. Wright on the Polarization 



drawn representing all the values in the Table. The results 

 given in the Table correspond very well with those obtained by 

 Professor Pickering*, who verified his values experimentally, 

 and showed that the deviation from theory in the case of four 

 plates only becomes perceptible above 65°. As Professor Pick- 

 ering used the value /z= 1*55, the numbers in his Table are 

 slightly greater than those used in constructing the curve from 

 Professor Adams's Table. 



The determinations were made by observation of the percent- 

 age necessary to render the bands visible with the same distinct- 

 ness as in the zodiacal light. A set of experiments were made 

 with light from the clear sky in a moonless night, the instrument 

 being directed to one of the brightest points of the galaxy, 

 where the light, though less bright than that of the zodiacal 

 light, did not very greatly differ from it in intensity. The glass 

 plates being turned until the bands had the same degree of di- 

 stinctness as in the previous observations, the mean of several 

 observations gave as the polarizing angle 41°, corresponding to 

 a percentage of 20*5. This value, on account of the inferior 

 brightness of the light compared, is somewhat too large, and 

 may be taken as an upper limit. 



To find a lower limit and, at the same time, an approximate 

 value, light reflected from a nearly white wall with a dead sur- 

 face was employed. The point observed with the instrument 

 was so chosen as to be equally distant from two gas-flames so 

 placed that the planes through them and the axis of the po- 

 lariscope were at right angles, thus giving light entirely free 

 from polarization. The flames were now turned down equally, 

 so that the field had, as nearly as could be estimated, the same 

 brightness as it had with the zodiacal light. A small scratch 

 upon the quartz plate, which could just be seen by the light of 

 the latter, served as a means of control in adjusting the inten- 

 sity. The experiments being conducted as before, gave, as the 

 mean of numerous determinations, the angle 36 0, 6, correspond- 

 ing to a proportion of 16 per cent., which is probably not far 

 from the true value of the amount sought. Another, in which 

 the light was made perceptibly brighter than that of the zodiacal 

 tract, gave for the angle 28°*5, and a percentage of 9'4, which 

 is certainly too small. We may safely take 15 per cent, as near 

 the true value. 



The fact of polarization implies that the light is reflected, 

 either wholly or in part, and is thus derived originally from 

 the sun. The latter supposition is fully confirmed by various 

 spectroscopic observations, of M. Liais f, Professor C. Piazzi- 



* Silliman's American Journal, S. 3. vol. vii. p. 102. 

 t Comptes Rendus, 18/2, vol. lxxiv. p. 262. 



