Harding. — Observations on Rainbows. 449 



in Napier during the same rainy season, but on no occasion 

 did I see both at the same time. 



In Wellington, last June, seeing a fine bow, I looked to 

 see if it presented any peculiar feature. It was only on close 

 examination that I detected that this one was double ; and, to 

 my surprise, I saw what I had not noticed before, that in 

 the second spectrum, forming the inner portion of the bow, 

 the colours followed the same order as the outer and brighter 

 series. I failed to see any colourless space between to repre- 

 sent the invisible portions of the spectra, and thence inferred 

 that they must have to some extent overlapped. 



I do not attempt to account for these variations in the 

 appearance of the bow, but should now scarcely be surprised, 

 under more than usually favourable conditions for observa- 

 tion, to see at the same time three double rainbows in the 

 heavens. 



Postscript. — Mr. T. B. Harding, to whom a copy of the 

 above note was sent, writes, — 



" On reading your paper, I could not account for the 

 doubling of the spectrum ; but I have since met with a scien- 

 tific note which I think may afford an explanation of the cause. 

 It appears that often the larger drops of rain are not solid, 

 but hollow spheres, as they are found to mark what they fall on 

 in rings [o] , while the others make spots [•] . Now, it seems 

 evident that, as a glass globe empty, or, rather, filled with air, 

 will refract differently from one filled with water, so the two 

 classes of raindrops will give differing spectra (in position), 

 yet, as we may imagine, so close together as to appear con- 

 tinuous. 



" With regard to the third rainbow, I do not see how it 

 could be formed in the ordinary way, as it would need three 

 reflections within the sphere, and two refractions, one on 

 entering and one on leaving. But there is another way in 

 which they may be formed — as a diffraction spectrum. I 

 saw one of them on Friday afternoon, the 11th November. 

 The sky where it was was clear. The arch was about 60°, 

 and resting at the ends each on a cloud. Turning towards 

 the sun, it appeared covered with a granulous cloud which 

 acted as a kind of grating or diffracting medium, and the 

 effect was perfect. It was not raining where I was, and did 

 not appear to be where the bow was seen. 



"I do not expect that you will ever see three double rain- 

 bows at the same time." 



29 



