218 Proceedings of the Royal Irish Academy. 



XXXIY. — XoTE oir THE Spectetjm, Polaeizatioj?-, and Foem of the 

 Zodiacal Light; as observed ijsr the Yeaes 1874 ais'd 1875. By 

 V,. E. BuetojS', B. a., Membee of the Eodeiguez Teansit of 

 y^ YEiftrs Expedition (Beitish.) ("Witli Plate 25.) 



[Read June U, 1875.] 



In the year 1872, the Royal Irish Aeaderay entrusted me with a 

 pecuniary grant which was to be applied in defraying the cost of con- 

 struction of a spectroscope, which should be specially adapted to the 

 examination of the extremely faint spectra of the Aui'ora and Zodiacal 

 Light. ( Vide Eeport on a Spectroscope of the binocular form.'*) 



An opportunity of studying the phenomena presented by the Zodia- 

 cal Light, under more favourable circumstances than those which 

 prevail in comparatively high latitudes, was afforded to me by my 

 being attached to the observing party sent by the British Government 

 to the Island of Rodriguez, situate in the South Indian Ocean. 



This fact having been represented to the Royal Irish Academy, and 

 their permission having been obtained to the employment of their in- 

 strument during my service with the Transit of Yenus Expedition 

 alluded to above, I was enabled to take advantage of several vivid dis- 

 plays of the Zodiacal Light which occurred while I was absent from 

 the United Kingdom, and to obtain the results detailed in the subse- 

 c[uent sections of this note. 



The observations will be entered in the following order : 



(1.) Those indicating the projected form of the luminosity : 



(2.) Spectroscopic results : 



(3.) The indications of sensible polarization : under each date on 

 which observations of the Light have been made in more than one of 

 the above mentioned respects. 



Date and, Place of Observation. — 1874, June 10*^, 9'^, 30™, approximate 

 local mean time. Lat. 32° X., Long., 14° W. 



Form. — The Zodiacal Light manifested 'itself very distinctly as an 

 ill-defined luminosity of an approximately triangular form, six or seven 

 degrees in breadth at the horizon, about forty degrees in length, 

 and inclined some fifteen degrees to a vertical circle bisecting its 

 base. 



Spectrum. — The spectroscope, on being directed to the bright cen- 

 tral region of the luminosity, showed a broad and nearly continuous 

 spectrum, sharply terminated on the less refrangible side, and fad- 

 ing gradually into complete darkness as its refrangibility increased. 

 The limits of the spectrum seen are defined by the wave lengths 



* Antca, p. 42< 



