6n the limits of single visiojf. 15i 



^decomposition of light by the irises they produce. It is ndt 

 thus with the eyt. I haVe given very great obliquity to the 

 rays I udniittied into it, without perceiving- any colour; and 

 if in some expe'rinients of artificial squinting I have thought 

 1 found slight indications of the decoftiposition of light, I 

 believe they must be ascribed rather to the changes pro- 

 duced iu the form of the humoUrs of the eyie by compres- 

 sion, than to any other causie. 



These proofs of th& perfection of the achromatic systerii 

 of the eye. established by experiment, and agteeabh; to the 

 general opinion of natuial philosophers, had seemed to me 

 secure from all doubt. I confess therefore I \*'as not a little C ntrary bpi- 

 isurpnsed, to find the contrary opinion maintained by IJ''' ed by Dr. 

 Toung in the paper I have quoted. This gentlemati says, Y^^"g' 

 it has been too lightly presurtied, that one of the final causes , 



of the structure of the eye was to render it achromatic: and 

 to this hypothesis he objects several experiments, from which 

 he thinks we may conclude, that the eye is not adapted to 

 unite all the elements of light in one point, as has been 

 lasserted. Art opinion adopted by Euler, and embraced by 

 all natural philosophers, should not 1 think be given up, till 

 the arguments opposed to it have been duly weighed. 



The first is drawn from the experiments of J urine on in- jurlne said to 



distinct vision. He is said, to have obsert^ed colours on the have observed 



, ,. , . • !• .■ . .1 t^ ■ . . , • m colours on the 



borders Of objects seen indistmfctly. It is true in his Trea- borders of ob- 



tise on distinct and indistinct Vision, printed in Smith'sJ''<=^5 s^^'^ ''^" 

 Optics, he speaks of the penumbra, that surrounds objects ^*^'"'^ 

 s-een indistinctly, but I have looked in vain for the observa- 

 tion of the irises in question. To satisfy myself of their 

 existence, I examined by daylight, and nearer than 'the fo- 

 cal distance of my eye, different bodies, and surfaces of 

 different tolours. With the coloured surfaces I constantly The author 

 observed the penumbra^ or circle of dissipation, as Jurine ^^^^ ""'> * ''^ 



• culls themi rOund these surfaces. This penumbra always 

 appeared to me to be formed of the colour of the surface 

 growing fainter from the centre to the circumference, but I 

 did not observe an iris round any of them. Things of lit- Small bodies 

 tie bulk, as small shot, slender wires of different metals, ^h"^^*^ ^''g^* 



•little bits of sealingwdx, resin, marble, wood, &c., observed the decomjio- 

 with due precautions, and at the least possible distance as ^^^''^'^ *^^ ''S*^*^ 

 K 2 12 



k 



