454 Mr. James C. M c Connel on the 



On these lines Sir George Airy** worked out the true 

 theory ; showing not only that the light attains a principal 

 maximum in one direction and a series of lesser maxima in 

 neighbouring directions, but also that the principal maximum 

 does not, as Young supposed, fall exactly in the direction of 

 minimum deviation. The amount of its divergence is de- 

 pendent on the diameter of the drops. The smaller the drops 

 the smaller is the rainbow, and the further are the supernu- 

 merary bows separated from the principal bow and from each 

 other. 



When the sun is shining on fog or mist, a bow is often 

 seen differing considerably in appearance from the ordinary 

 rainbow — far broader and, though quite bright, nearly colour- 

 less. The breadth may be as much as 6° or 7° instead of 1^° 

 as in the rainbow, and the colouring seldom surpasses "a 

 faint dull red and orange at the outer border and a slight blue 

 tinge " at the inner. The mean radius is a few degrees less 

 than in the rainbow. Verdetf considers this to be a degraded 

 form of the primary rainbow. Accordiug to Airy's theory 

 the radius would be diminished by smallness of the drops; 

 and Verdet attributes the whiteness (1) to the faintness of 

 the illumination, and (2) to the irregularity in size of the 

 drops. There can be little doubt that the main contention is 

 correct, and more satisfactory reasons may be adduced for the 

 want of colour, as we shall see. 



At the top of Ben Nevis there are many opportunities for 

 seeing fog-bows, and the observers there are provided with a 

 special instrument"]: for taking their angular dimensions. 

 Mr. Omond has kindly sent me a list of such observations 

 from May 1886 to October 1887, extracted from the Proceed- 

 ings of the Royal Society of Edinburgh for 1887. Those 

 relating to fog-bows are quoted below. The temperatures 

 (Fahr.) were taken in a Stevenson screen at the times named. 



(1) " October 7, 1886. Temp. 39°'8. At 12 h. a solar fog- 

 bow. No colours, only a broad white band. 



Radius to inside of bow . . . 36 20 

 „ outside „ ... 43 36 



(2) "October 22. Temp. 31°-5. Fog-bow seen at 11 h. 

 25 m. Colours as in fig. 1." [They are, from the outside 

 inwards, brownish red, pale green, pink, light blue, red, 



* Trans. Camb. Phil. Soc. vi. p. 379 ; viii. p. 595. 



t Lecons d'Optique Physique, i. p. 422. 



\ This instrument, named a stephanoine, consists of a graduated bar, 

 at one end of which the eye is placed and on which slides a cross bar 

 carrying certain pointed projections. With its aid faint objects, for 

 which a sextant would be useless, may be measured to within 5'. 





