55 ACCOUNT OF TWO HALOS. 



A Theori/ of Halos and Parhelia. By Th omas Young, 



M. D. F. R. S. 



Theory of halos The explanation of the primary and fecondary rainbovr 



and parhelia. begun by De Dominis, and completed by Defcartes and 



Newton, derives an entire and fatisFaflory confirmation, from 



the perfe6l coincidence of the obferved angular magnitudes, 



with the refult of calculations of the effeft of fphericai drops. 



We know that drops of water, either accurately, or very 



nearly fphericai, exift in great abundance in every cloud, and 



in every fliower of rain ; and whatever their dimenfions may 



be, they mufi: neceflTarily confpire in the fame general effefl, of 



producing the fame rainbow, whenever a fpedator is placed in 



a proper lituation for obferving it; confequently fuch rainbows 



are of very frequent occurrence. 



Variable halos I have attempted to (how, that for producing the phenomena 



produced by ^^ variable halos, often obfervable in hot climates, it is only 



equal drops, . , -' 



neceflary that a confiderable part of the fpherules of a cloud or 



mift, be either accurately, or very nearly, of equal magnitude, 

 a condition, of which the poffibility is eafily admitted from 

 analogy, and the probability is favoured by the apparent unifor- 

 mity of the different parts of fuch mills as we can examine. 

 Theconftant But no fatisfadory reafon has hitherto been afligned for 



halo of 23* not (^q produdion of the halo, which in thefe climates is the 

 moft common of all j that is, the conftant halo of 23** or 2t°. 

 The hypothefis by which Huygens attempted to explain the 

 production of halos and parhelia, are both arbitrary and im- 

 probable. He imagined the exigence of particles of hail, 

 fome globular, others cylindrical, with an opaque part in the 

 middle of each, bearing a certain ratio to the whole; and he 

 fuppofed the pofition of the cylinders to be fonietimes vertical, 

 and fometimes inclined to the horizon in a given angle. 

 The hypothefis It has already been objeded, that no fuch particles have 

 b^bP^"' ^"™' ^^^"^ hcen obferved to accompany halos ; and it is, betides, 

 highly improbable, that fuch an opaque part fliould bear the 

 fame proportion in all the hailftones, and that the cylinders 

 fhould have terminations fo peculiar as is fuppofed ; and the 

 moft incredible circumftance of all is, that all thefe proportions 

 fhould be conftantly fuch, as always to produce a halo at the 

 diftance of 23° or 24° from the fun or moon. 



It 



