A complete physico-medical and chirurgical treatise on the human eye / [Peter Degravers].

  • Degravers, Peter
Date:
1788
Catalogue details

Licence: Public Domain Mark

Credit: A complete physico-medical and chirurgical treatise on the human eye / [Peter Degravers]. Source: Wellcome Collection.

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    7z j PROPERTIES OF ought to have generally experienced, that you are not able to difcern any object at all for a confiderable time. In what manner ought we to account for this fact ? If you pafs from a dark room into a well lighted one, all the objects,which furround you, fend immediately fuch a quantity of Among rays on the retina, that your re- flection is quite taken away by their too violent im- prefiion ; becaufe, the fibres of the iris not being fo quick in their action to operate a motion of extenfion, which produces a contraction, as the introduction of the rays into the globe, the too gre^t quantity of thefe rays cannot be intercepted in time, to become in pro- portion to the fenfibility of the immediate organs of fight : If, on the contrary, you pafs from a well light' ed room into a dark one, ths contraction in the pupils, neceflary to moderate the ftrength of the rays, when in the firft room, operates fuch an interception of them at the time you arrive in the fecond. that the number is not fulficient to operate a fenfation on the retina, as they are emitted from objects which receive tliemfelves no fulficient power from the primary light ; becaufe you are obliged to ftay a long time in the dark room, to give the fibres of the iris a proper time of operating a dilatation, in order to admit a Sufficient quantity of rays on the immediate organ of .right, capable of pro- ducing a fenfation upoii it. There are fome eyes, I know, whofe pupils are more iufceptible of con- traction and dilatation than others ; but this mult be out of the general rules I am now fpeaking of. If, now, we are to form a conc]ufion,fcom all thefe obfer- vations, without any further reafoning about them than fuch as they juftify, what mu ft it be ? — It muft be plainly this, that there is in the whole animal kind one intelligent fpring, common to every fpecies, but vaftly diftinguiffied in its efieds j that tho* it
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