114 EXPERIMENTS, &c. 



rays which proceed from objects at every di- 

 stance whatsoever, not less than ten inches, and 

 even such as are somewhat convergent, since it 

 can make those, which are parallel, to meet be- 

 fore they fall upon the retina ; and lastly, that, 

 while both the optic axes are directed to a 

 point within the limits of distinct vision, the 

 rays proceeding from it are never accurately 

 collected to focuses in both retinas, and scarcely 

 ever to a focus in either retina. These are like- 

 wise the > principal circumstances, in which my 

 experiments differ in their results from those of 

 Dr. Porterfield. 



In making such experiments with luminous 

 points, one or other of two appearances very 

 constantly occurs, neither of which, as far as I 

 know, has been spoken of by any preceding 

 author. The most proper way of mentioning 

 what they are, is, perhaps, to show what ought 

 to happen in those situations, in which they 

 are observed. 



When a beam of white light passes, obliquely, 

 from one medium into another of different re- 

 fractive power, its variously coloured rays must 

 begin to diverge from each other, at the point 

 of the beam's incidence upon the latter me- 

 dium. In achromatic telescopes, the mutual 

 separation of these rays is checked, and its 

 farther increase prevented, before it becomes 



