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Many rays pass into the drop; but only those will be parallel and produce an effect on the retina, which are incident in the vicinity of that point on its surface fig 233 where, from the nature of the medium of which it consists, the incident ray R A makes the greatest angle with the emergent ray DE. If the situation of the drop of moisture is such, that coloured rays emerging from it nearly parallel, will enter the eye of an ob- server, he will perceive the correspond- ing colour. The greatest angle in rain water for red rays, emitted after one reflection, is 42 30: and for violet, 40 30 Hays like R" incident on other parts of the surface of the drop, emerging in the direction ZQ, will he so divergent as not to enter the eye. 165 Let, therefore, ET be a line passing from the sun through E the eye of the spectator, and by consequence parallel to RA : and let TED 40 30 If ED revolves on ET as an axis, D will describe the circumference of the base of a cone ; and any rays in the direction RA, which, during any part of its revolution, fall on the corresponding drop, will emerge in a direction E ; and, since the angle between the incident and emerging rays =40 30, they will be among those violet rays which, emerging nearly parallel, reach the eye, and produce on the retina the sensation of that colour. In the same way, if the angle TED = 4 2 30, the emerging rays will belong to those red rays which, being nearly parallel, reach the eye, and pro- duce the sensation of the latter colour. prev next |