tfiS Dr, Ni$holl on a peculiar Lnp6rfectio?i [Feb ^ 



domorphose forms. I have also been unable to ascertain its 

 presence in the white botryoidal si/icates of zinc from Hujigari/, 

 But as this article has already been extended to a greater length 

 than I at tirst expected, I shall, for the present, postpone making 

 any further observations. 



Edward Daniel Clarke. 



Article XII. 



Remarks on a peculiar Imperfection of Vision with Regard to 

 Colours, By Whitlock Nicholl, MD. MRIA. FLS. &c. &c. 



(To the Editor of the Annals of Philosophy.) 



SIR, 



Cases of imperfection of vision with regard to colours are 

 recorded in the Philosophical Transactions ; and similar cases,* 

 which I have reported, have been published in the Transactions 

 of the Medical and Chirurgical Society of London.f 



The principal peculiarities in each of these cases are ; the 

 confounding red with green, and pink with blue. Since no 

 attempt has (as far as I am acquainted) been made to explain 

 the cause of these peculiarities of vision, I am tempted to hazard 

 some conjectures respecting it. 



Before I enter, however, on the subject of these cases, I must 

 premise a few general observations. 



Cases occur in which the sensibility of the retina, with regard 

 to light, is above the natural standard. We also meet with 

 cases in which that sensibility is below that standard. In the 

 former cases, a faint degree of light causes the production of 

 that sensation which we term seeing ; whereas, in the latter 

 instances, a strong degree of hght is requisite for the production 

 of that sensation. A retina, then, may be sensible to a powerful 

 degree of Hght, although it is insensible to fainter degrees of 

 light. 



When the retina has been powerfully acted upon by a power- 

 ful degree of hght, it may become insensible to light; or, if it 

 have been accustorhed to the action of strong degrees of light, it 

 may become insensible to fainter degrees of light ; whereas a 

 retma, from which all strong degrees of Hght have been -for 

 some time withheld, may become sensible to very faint degrees 

 of light. 



When the retina has been powerfully acted upon by one parti- 

 cular set of the prismatic rays, it may become insensible to that 



• See two papers, the one bearing date Jan. 1 777 ; the other, May in the wune year, 

 f Vols. vii. andix. 



