A 



JOURNAL 



OF 



NATURAL PHILOSOPHY, CHEMISTRY, 



' ; AND 



THE ARTS. 



SIR, 



JULY, 1807. 



ARTICLE I. 

 To Mr. NICHOLSON, 



J3J OTWITHSTANDING the very ingenious inveftiga- Looming, or 

 tions of Dr. Wollaston and others, it appears to me, that h°" z °ntal re- 



- , . „ , . , . if.- • ■,, fraction. 



the subject ot looming, or horizontal retraction, is still ca- 

 pable of being explained with greater precision, and upon 

 simpler principles: I shall therefore trouble you with a few 

 observations, which have occurred to me respecting it. 



Let the refractive density of a medium be supposed to Suppose strata 



vary "Tadually and equally, in parallel strata; the variation regularly. <Jimi- 

 ; . . .- ; „ ' . . rushing in re- 



begmmng, from a certain plane surface, and being conti- fractive power. 



nued, till, at a certain distance above that surface, the re- 

 fractive power wholly vanishes. For example, the re- 

 fractive density of air being expressed by 1.0003, if the tem- 

 perature vary 1° in 1 foot, the refractive power will vary 

 .000 000 6; and dividing 1.0003 by this, we have 1 666 667 

 feet, for the imaginary height of a medium continuing to 

 vary at the same rate till its refractive power vanishes. 



Now upon the projectile hypothesis, supposing a particle Progress of 



of light to be initially at rest in this medium, it will be ac- hght lhrou S h 



J it. 



tuated by a constant accelerating force; and by falling from 



the top to the bottom, it will acquire the velocity natural 



to light in the original medium : and if a ray of light enter 



' Vol. XVII. —July continued, 1807. M the 



