134 



Dr Young's 

 experiment of 

 a triangular 

 spectrum from 

 a uuiinous 

 Doint, 



repeated by 

 the author on 

 j)ouUs it very 

 differ f-ul di»- 



the figure of 

 the specirura 

 varieil by 

 changing the 

 inr linjtionof 

 the prism. 



The sxperV- 

 meru not de- 

 cisive. 



ON THE LIMITS OF SINGLE VISION. 



rays that graze tlieir edges are decouipoKed; and this de-» 

 compogiiion, which is not perceived at a distance pn account 

 of the m -^ture of the very numerous unaltered rays, is disr 

 tinguished when very <Iose, because they reach the retina 

 ahnost without mixture. 



Dr. Youns^ likewise o]iposes the followinsf experiment to 

 the theory of the perfection of the achromatic system of the 

 eye. Having received on a prism the rays issumg from a 

 luminous point, he ohyerved, that, instead of obtaining^ a 

 linear spectrum, as tiie theory of the equal reiraug'ibility of 

 the different rays by the humours of the eye would lead us 

 to expect, the spectrum appeared of a triangular tigure; so 

 that, if the eye adapted itself so as to unite the red rays in 

 one point, the blue would be refracted, a^d vice versa. 

 From tliis fact he infers the unequal refrangibility of the 

 different luminous ray^ by the humours of the eye. This 

 experiment of Dr. Young I have repeated with great care, 

 and on different kinds of lun)inous points, as on the light of 

 a candle seen at 4 met. [13 feet] distance thr-agh an aper- 

 ture 5 rail, [near 2 lines] in diameter, that of the public 

 Jai^terns from 400 to (JQO [440 to 660 yards] distance, and 

 that of the hxed stars in a clear night. I observed the 

 triangular figure mentioned by that gentleman, whenever I 

 gave a little magnitude to the spectrum ; but in proportion 

 as I increased its dimensions in length, by giving a greater 

 inclination to the refracting surfaces of the j)rism, thistigure 

 diminished, and t\\e spectrum apprqached more nearly to a 

 parallelogram, the middle of which however continued nar- 

 rower than the extremities. The end occasioned by the 

 blue rays appeared to me generally broader than that pf the 

 red; but what particularly surprised me in this experiment 

 was the tiguie of a doqble fan, the radii of which crpssed 

 each other in the centre, taking their course apparently in 

 difftrent planes. 



Though this ejcperiment seems tq iiidicate an unequal 

 ppvyer of refracting the different luminous rays by the hu- 

 lyiours of the eye, it is not absolutely such as the author de- 

 scribes it, and a fact so complicated seems to tne little 

 adapted to elucidate this theory. To conclude thatth^ hu- 

 IftOMrs of the pye are not perfectly achromatic, the It^- 



minou^ 



