Prof. Haidinger on the Polarization of Light by Vapour. 55 



small elevation through the window into the vapour. A beau- 

 tiful circular arch presented itself to the eye, the centre of which 

 was the shadow of the head. I endeavoured to represent it in 

 the diagram fig. 1, A B CD being the projection of the window 

 upon the wall on the opposite side of 

 the room. 



" The colour of the arch / is a pale 

 bluish white. It is slightly fringed on 

 both sides with a pale orange or brown- 

 ish yellow, not over bright. The 

 space e without and the space y within 

 the arch is inferior in light, and of a ( 

 grey, rather reddish colour. Opposite 

 to the eye, the sun just grazing the eye, 

 there appears a brighter circular spot a, 

 fringed at b with the slight yellowish 

 or reddish tint. Beginning from£,the light is distinctly polarized. 

 The brushes of polarization are quite visible if the eye from one 

 place or direction is quickly directed to another. The brushes 

 have a direction corresponding to the radius in the whitish arch, 

 and a tangential direction in the spaces within and without it. 

 The light of the arch appears, then, to be polarized by reflection 

 from the surface of the particles of vapour or water. The spaces 

 without and within the arch appear, therefore, to be polarized by 

 transmission perpendicularly to the polarization of the arch. The 

 bluish-white and the reddish tints may be faint mixtures of the 

 bluish or reddish fringes of diffraction, combined with the direct 

 reflection from the watery particles floating in the air. 



" It is well known that a real rainbow may be produced on a 

 small scale by taking some water in the mouth and then forcibly 

 spouting or puffing it out reduced to the finest watery dust or 

 powder. I availed myself of this method to ascertain, at least 

 approximately, the diameter of the nebulous arch, being without 

 any other apparatus in a vapour-bath. The nebulous arch still 

 continued visible, as in fig. 2 ; but the first or interior rainbow 

 now became visible, and was situ- 

 ated pretty much in the central line 

 of the nebulous arch ; the exterior 

 rainbow, visible only in faint traces, 

 appeared beyond the nebulous arch. 

 The angular values of the semidia- 

 meters being for the red of the in- 

 terior rainbow 42° 2', for its breadth 

 1°45', for the red of the outer rain- 

 bow 50° 58', and its breadth 3° 10', for the distance of the two 

 rainbows 8° 15', the breadth of the nebulous arch is consequently 



Fig. 2. 



