An essay on visual glasses, (vulgarly called spectacles) wherein it is shewn ... that the common structure of those glasses is ... very prejudicial to the eyes; the nature of vision in the eye explained, and glasses of a new construction proposed / [Benjamin Martin].

  • Martin, Benjamin, 1705-1782.
Date:
1757
Catalogue details

Licence: Public Domain Mark

Credit: An essay on visual glasses, (vulgarly called spectacles) wherein it is shewn ... that the common structure of those glasses is ... very prejudicial to the eyes; the nature of vision in the eye explained, and glasses of a new construction proposed / [Benjamin Martin]. Source: Wellcome Collection.

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    I ( 19 ) Quantity of Light conftantly refraded and thrown upon the Eye-Balls by fuch large Glaffes, muft naturally tend to render them weak and watery by Degrees $ and this Effed we generally fee attends the Ufe of common Spectacles. Befides, that Reafon in all Cafes forbids the Ufe of any Thing farther than is proper to anfwer the End propofed $ and fince by all Experience it is known, that too much Light is prejudicial in forming the Image of Objeds, it muft be abfurd, in the higheft De¬ gree, to admit of more than is fufficient or ne- ceffary for that Purpofe in the Eye where Vi- fion is required to be moil: of all perfed. Again, it is contrary to Nature 5 for we ob- ferve in the Eye a particular Provifion made for adjufting a proper Quantity of Light for diftind Vifion, in the Aperture of the Pupil $ and by this the Aperture of the Spectacle- Glaffes fhould be limited, that is, they fhould be fo large as that the Pencil of Rays from any Point A, refracted through them, fhould be fufficient to fill the Area of the Pupil $ and if there be any more, as it can’t enter the Pupil, it muft do harm to the other Parts 5 and therefore ought to be excluded. By this Means we ffiall find the Area of common Spedacle-Glaffes near three times as large as it ought to be. And, thirdly, it is a Pradice contrary to Art 5 for who does not, or ought not, to know, that in the Structure of a Microfcopey Telefcope, it is abfolutely neceffary that C 2 there