-Dr. Hosack’s Observations on Vision. igg 
retina ; on this union of the rays at the retina depends distinct 
vision. But supposing the eye of a given form, should the 
point of union lie beyond the retina, as must be the case with 
those from the less distant object, agreeable to the preceding 
proposition ; or should they be united before they arrive at 
the retina, as from the more distant object, it is evident that 
the picture at the retina must be extremely confused. Now 
as the rays which fall upon the eye from radiating points at 
different distances have different degrees of divergence, and 
the divergence of the rays increasing as the distance of the 
radiating point lessens, and, vice versa, lessening as that in- 
creases ; again, as those rays which have greater degrees of 
divergence, viz. from the nearer objects, require a stronger 
retractive power to bring them together at a given distance 
than what is necessary to make those meet which diverge 
less, it is manifest, that to see objects distinctly at different 
distances, either the refractive power of the eye must be in- 
creased or diminished, or the distance between the iris and 
retina be varied, corresponding with the different distances 
of the objects ; ootn of which probably take place, as will 
hereafter appear.* 
Having then established these as our premises, we shall 
next examine the different principles which have been em- 
ployed for explaining vision at different distances. 
* “ Facile enin intelligitur, quo longius radii adveniunt, eo magis esse parallelos ; 
“ eo minus ergo differre ab axi, et eo minoribus viribus cornese et lentis crystalline in 
“ focum cogi. Ut enim corpus magis distat, ita sub minori angulo radii adveniunt. 
“ Contra si corpus conspicuum valde vicinum fuerit, radiorum ab eo advenientium an- 
“ £ ulus est major, et adeo magis divergences in oculum incidunt, et viribus egent refrin- 
“ gentibus majoribus omnibus densioribus.”— Haller. Elem. Phys. lib. xvi. 
