ON THE LIMITS OF SINGLE VISION. 133 



experiments made with the optometer. This is the name meternrgedby 

 Dr. Young gives to an instrument intended lo find the focal ' Y"""«- 

 distance of the e3'e, or focus of distinct vision. The instru- 

 ment is composed principally of a thia slip of some sub- 

 stance with two very small apertures, through which objects 

 are to be looked at in a certain position. The iiises that 

 exhibit themselves at the surface of bodies seen through 

 these small apertures are considered by the learned English- 

 man as produced by the unequally refractive power of the 

 humours of the eye on different luminous rays. Howtver, 

 to substantiate this inference it should have been demon- 

 strated, that the decomposition of the light tannot proceed in these the 



from any other cause : yet here the cause is evidently' exter- ''f^'^^ <lecom- 



11 1 • r I r • i^ , posed by tauses 



nal to the eye, and independent ot its action. Since the independent of 



time of Grimaldi all natural philosophers have acknow- ^^^^ ^y^- 

 ledged the action of bodies on the luniitious rays, that graze 

 or approach their surfaces; and Newton has shown by expe- 

 riments of the greatest accuracy, not only that light is at- 

 tracted by biklies, and inclines toward their surfaces, but 

 also that this attractive power of bodies acts with a different 

 force on the different rays. Now in the present iastauce we 

 look at bodies through small apertures ; so that the lumi- 

 nous rays, by means of which we see them, necessarily 

 experience the action of the edges of the apenures they 

 pass through, and must thus be decomposed. 



To convince ourselves, that the irises are produced by this Experiments 

 cause, it is sufficient to look at a di'^hmt object, the frame "^ i'^°^* *^^** 

 of a window for instance, at the same time intercepting- by 

 means of a cord, the edge of a knife, or some other thin 

 substance, part of the rays that would arrive at the opening 

 of the pupil; when we shall perceive the object terminated 

 by an iris, and this iris changing its position, according as 

 ♦he edge of the interposed substance is vertical or horizon- 

 tal, inward or outward, &c. And what other cause can be 

 assigned for this phenomenon ? Why should the eye de- 

 compose the rays, that reach it through small apertures, and 

 not decompose those that reach it without such intervention ? 

 This explanation of irises observed at the surfaces of ebjects 

 Been through sn;all apertures is equally applicable to the 

 faint irises observed iu objects viewed iudiitlnctly. The 



rays 



