APPENDIX B: ROGER BACON 405 



lighter parts red and orange. But we see the same colours in the 

 dew drops, where there can be no such differences of density ; simi- 

 larly in the crystal. Aristotle has been wrongly translated and 

 interpreted in this matter. Another erroneous belief is that lunar 

 rainbows occur only once in fifty years. They may occur at any full 

 moon under suitable atmospheric conditions. 



CHAPTER XI 



The shape of the bow is a difficulty. It cannot be explained by 

 refraction. It is to be observed that the same colour is continued all 

 round the circle in each ring. All parts of the ring therefore preserve 

 the same relation of the solar ray to the eye. This implies circularity 

 of form. It is asked why the whole space contained by the circle is 

 not occupied with colour. Because from the points in this central 

 area rays equal to the angle of incidence are not reflected to the eye. 



CHAPTER XII 



The cloud therefore is not coloured ; the appearance of colour, for 

 it is only an appearance, is given by rays reflected from the raindrops. 

 Of colours there are five, white, blue, red, green, black ; though Aris- 

 totle, dividing blue and green into other shades, speaks of seven. 

 These colours appear to have some relation to the various structures 

 of the eye. In addition to the problem of the rainbow, there is the 

 problem of haloes and coronal. On this I give the best explanation 

 that as yet occurs to me. I do not however, pretend that it is satis- 

 factory. Far more careful experiments, made with properly con- 

 structed instruments, are needed before an adequate explanation 

 can be given. 



THE SECOND PREROGATIVE OF EXPERIMENTAL SCIENCE 



In all sciences Experiment is able to reveal truths quite unconnected 

 with the discussion of principles, and with regard to which it is useless 

 in the first instance to assign a reason. The initial state of mind 

 should be readiness to believe ; this should be followed by experiment : 

 reasoning should come last. I subjoin examples of my meaning. 



1. The astronomer constructs his spherical astrolabe, by which he 

 can observe the precise longitude and latitude of heavenly bodies 

 at different times. But it is not inconceivable that experiment may 

 devise means of bringing this instrument into such relation with the 



