460 PHILOSOPHICAL TRANSACTIONS. [aNNO I67O. 



ent dispositions all the appearances of the parhelia and their circles did neces- 

 sarily follow. 



And first, that some of these cylinders being erect, in the situation which 

 probably they ought to have in being formed, there must appear in the heavens 

 a large white circle^ parallel to the horizon, passing through the sun, and of 

 near the same breadth with him ; as has been observed in the phenomenon of 

 Rome, An. 1629, of which Gassendus and Descartes have written, and which 

 is here exhibited in fig. 5. That this circle L K N M is caused by the reflec- 

 tion of the rays of the sun on the surface of these cylinders ; it being easy to 

 demonstrate, that there are none but those which are raised at the same angle 

 above the horizon with that of the height of the sun, that can reflect his rays 

 to our eye. Whence it manifestly follows, that it must appear white, and 

 throughout of equal altitude with the sun himself, and consequently parallel to 

 the horizon. That considering afterwards the transparency of these perpendi- 

 cular cylinders, and their opaque kernels, it is easily seen that those of the 

 white circle, which are distant from the sun at a certain angle, begin to give 

 passage to his rays to strike our eyes, in the same manner as has been said of 

 the round half-dark grains. That these cylinders are those which on each side 

 of the sun make us see a parhelion in the large white circle, as has been noted 

 in the observation of Rome (where they are marked with K and N) and in 

 many others. That these parhelia have commonly luminous tails ; because the 

 cylinders which follow those first ones that form the parhelia, and which are 

 yet further distant from the sun, let also pass his rays to our eye, so that these 

 tails may be 20 degrees and more in length. That the same parhelia are 

 always coloured, because they are made by refraction, as the halo. 



That besides, there are two other images of the sun, generated by these 

 perpendicular cylinders, and so disposed in the large white circle, that the spec- 

 tator turning his face towards the true sun, has them behind him ; as in the 

 Roman observation are the parhelia L and M. That these are produced by 

 two refractions and one reflection in these cylinders, in the same manner as 

 the ordinary rainbow in the drops of water, according as M. Descartes has 

 declared : so that the opaque kernels do nothing to the production of these 

 two suns, but that they may be sometimes so large as to make them not 

 appear. That according to the altitude of the sun, more or less, these two 

 parhelia are more or less near to one another. That they should appear co- 

 loured, as the rainbow, and that sometimes they have been seen such ; but that 

 when they are faint they may also seem white, even as the halos when they 

 are not very bright. That these same perpendicular cylinders can also produce 

 a halo about the sun, by reason of the rounding of their two ends ; so that. 



