APPENDIX B: ROGER BACON 403 



from the centre of the sun through the eye of the observer and the 

 centre of the circle of which the bow is an arc to the sun's nadir. As 

 one extremity of this line is depressed, the other is elevated. It be- 

 comes thus possible to compute the altitude of the sun beyond which 

 no rainbow is possible, and also the maximum altitude of the bow. 

 It will be found both by calculation and experience that this altitude 

 in the latitude of Paris is forty- two degrees. 



CHAPTER V 



Still further investigating the shape of the iris, and the portion of it 

 that can be seen, the experimenter conceives a cone of which the 

 apex is the eye, the base is the circle of the iris, the axis being the line 

 already described drawn from the sun's centre through the eye to the 

 sun's nadir. In cases where this cone is very short, the whole of the 

 base may be above the horizon, as may often be seen in the spray of 

 a waterfall. In the sky, however, the cone is too elongated to admit 

 of this : the base is bisected in various proportions by the plane of 

 the horizon. The arcs visible are not portions of the same circle. 

 When the sun is high, and a small arc is visible, it belongs to a larger 

 circle than the arc seen when the sun is rising or setting. A bow can 

 be seen when the sun is just below the horizon ; but owing to terres- 

 trial vapours, only the crown of the arch is usually seen. 



CHAPTER VI 



In some latitudes there can be no rainbow at noon even in the win- 

 ter solstice. When the latitude (i.e. the distance from the zenith 

 to the equator) is 24 25', the sun's altitude at noon in the winter 

 solstice will be 42, therefore there can be no bow. Passing north 

 from this latitude, there can always be a noon rainbow till we come 

 to latitude 66 25', when at the winter solstice there is no sun. Similar 

 calculations can be made for other latitudes. 



CHAPTER VII 



We have now to inquire whether the iris comes from incident, 

 reflected, or refracted rays. Is the bow an image of the sun? Are 

 the colours on the clouds real? Why is the iris of circular form? 

 Here we call experiment to our aid. We find on trial that if we move 

 in a direction parallel to the rainbow it follows us with a velocity 

 exactly equal to our own. The same phenomenon occurs with respect 



