222 Proceedings of the Royal Irish Academy. 



Date. — 1874, August 12'*. Time not noted. 



Place of Ohservatioji. — At sea — between ITauritms and Eodriguez. 



Polarization. — With, the Savart arrangement hands were visihle, 

 which were sensibly parallel to the axis of the Zodiacal Light, when 

 at their greatest intensity ; and the plane of polarization indicated 

 seemed to lie in the same plane as the bands. 



A Nicol prism confirmed the restdt obtained with the Savart — the 

 mode of observation being this : the prism was rotated slowly until a 

 faint star involved in the Zodiacal Light appeared brightest. 



Pate.—Ul4:, October 10'' 7'^+. 



Place of Observation. — Eodriguez. Observatory House. 



Spectrum.- — The measures obtained give a wave length of 5655 

 seventh-metres for the bright line terminating the spectrum on the 

 less refrangible side. 



After October 10, the use of the recording spectroscope had to be 

 discontinued, to my great regret ; the cement used for securing the 

 prisms having been affected by the heat and dryness of the climate, and 

 become brittle and unsafe. 



The only substitute available (pure wax) proved equally unreliable. 



Pate. — 1875, January 2^ 8^ to 9'^ 30'", local mean time. 



Place of Observation. — Curepipe, Mauritius. 1800 feet above sea 

 level. 



Spectrum. — A pocket spectroscope with one compound prism of 

 direct vision showed a Zodiacal Light Spectrum identical with that 

 formerly obseiwed, except as regards the dark streak of wave length 

 5355, which was not detected. 



Polarization. — A Mcol prism, rotated about its longer axis, produced 

 marked variations in the intensity of the part of the Zodiacal Light 

 viewed, upon which I kept my attention fixed during the experiment 

 by retaining two faint stars involved in the luminous haze in the centre 

 of the field. These stars were barely visible to the unarmed eye, but 

 were well seen when the principal plane of the prism was at right 

 angles to the axis of the Light. 



The Savart arrangement also indicated polarized light, but much, less 

 decisively, as the Light was setting. 



Form. — HeiT Heidom, of the German Expedition to Mauritius to 

 observe the Transit of Yenus, joined me in making careful observations 

 on the general form and position of the luminosity. 



We noted that the northern boundary of the Light was much better 

 defined, and more nearly straight, than the southern ; also that it passed 

 close to a great circle described through 77 Tauri and ^ Pegasi. 

 The southern boundary nearly coincided with a parabolic curve passing 

 through a Tauii, ^ Ceti, and a Gruis. The luminosity extended from 

 the "W. horizon, where its breadth was about twenty-five degrees, cer- 

 tainly as far as the Milky Way in Taurus, its apparent width at the 



