small heavy Bodies in Air, &c. 255 



stances favour, either white, or of colours more or less dilute, 

 being internally red, and externally of the prismatic orders, or 

 mixtures of these. These colours, as they are more vivid, are 

 also more extended, even to twenty-six or twenty-seven degrees, 

 and appearing in narrow and long portions of vapour, exhibit 

 internal reds, yellows, and bright central mixtures of all the 

 rays, emulating the sun in brightness, and gradually decaying 

 away at increasing distances from him. One or two of these, 

 seen alone, or in conjunction with other less resplendent parts 

 of the iris, have received the appellation of mock suns. One of 

 the most remarkable appearances which attend this bow, to be 

 attentively observed, and to be carefully remembered in at- 

 tempting its explanation, is a dark circle, or circular shade, 

 appearing immediately within the preceding coloured circles. 



If a globe of glass, filled with water, be exposed to the sun's 

 light, or to the light of a candle, the rays which pass through 

 converge from different points of the second surface to points 

 in the line passing through the centre of the globe parallel to 

 the incident rays. This cone of rays, received on a plane at 

 right angles to the incident rays, exhibits a circle coloured 

 with bright red on its external border, brighter yellow adjacent 

 thereto, internally brightest white light within the yellow 

 becoming more and more dilute up to the axis of the cone. 

 On the external border of the red at the same time appears a 

 dark circle or circular shadow, obviously produced by the ex- 

 treme external parts of the globe intercepting all other light, 

 and the light incident on the globe being all turned aside 

 therefrom, and refracted entirely within the cone of passing 

 light. The exact agreement of the iris in all its parts with 

 these the phenomena of single drops, renders the conclusion 

 irresistible, that it is formed from drops, by two refractions 

 from drops of water. All that remains to be accounted for, is 

 the variation of the diameter from what drops of water in other 

 circumstances of other refractive powers would make it. 



The colours and the dark line on the external edge or limb of 

 the cone of light passing through the drops, prove that those 

 t olours come from those points in the drop wliich are the limits 

 S 2 



