On the Polarization of the Zodiacal Light. 13 



Now in Amsler's planimeter the point B is constrained to 

 move in the arc of a circle, while the pencil A is traced round 

 the contour of the required area ; this is simply a limitation of 

 the more general case taken above. Also the wheel whose 

 travel is measured is placed away from the centre of the bar C, 

 and indeed on the opposite side of B ; but, as we have seen, its 

 position, so long as its centre is on the line A B, is quite imma- 

 terial, its motion in the aggregate being the same as if it were 

 placed at C. 



In the planimeter the length / is capable of variation ; so that, 

 by setting it differently, the same graduation on the wheel will 

 give areas in different units, the unit of area being always Ix 

 the circumferential travel of the wheel required to alter its read- 

 ing by unity. 



III. On the Polarization of the Zodiacal Light. 

 By Professor Arthur W. Wright*. 



FROM the published accounts of observations upon the zo- 

 diacal light, it would seem that few attempts have as yet 

 been made to determine whether or not any portion of the light 

 is polarized, and the results thus far obtained leave the question 

 still undecided. The few notices that can be found in the scien- 

 tific journals, though uncertain and contradictory, tend to the 

 view that it is either not polarized at all, or that the proportion 

 of polarized light is so small as to render its detection a matter 

 of excessive difficulty. It may be observed that most of the 

 observations giving negative results appear to have been made 

 with Savart' s polariscope ; but with an instrument which ab- 

 sorbs so large a proportion of the light as a Savart, the amount 

 of polarization necessary to render the bands visible increases 

 very greatly as the light becomes fainter, and especially so as it 

 approaches the limit of visibility. Numerous attempts have 

 been made by the writer to detect traces of polarization with a 

 Savart, but never with the slightest result, excepting that on one 

 especially clear evening, when the zodiacal light was unusually 

 distinct, the bands seemed to be visible by glimpses, on the 

 utmost exertion of visual effort. The observation was so un- 

 certain, however, that it was considered worthless. 



Nearly a year ago a series of observations was begun, in the 

 course of which a variety of apparatus were employed, by the use 

 of which it was hoped polarization might be detected, either, as 

 in the Savart, by bands or other variations in the brightness of 

 parts of the field, or as with the double-image prism, the NicoPs 



* From Silliman's American Journal, May 18/4. 



