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].

  • Benjamin Martin
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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    parifon between the Form of vulgar Speftacle$ and thofe which are here recommended of a new and poilofophical Conftrndtion. To this End I have reprefented the two Eyes D X E. and DYE, at the Diftance they have in the Head with the Spedtacle-Glafles placed before them at the ufual Diftance alfo j the firft of which, G H, is that of a common Pair, placed right before the Eye DX E j the other G H is one of the new Form placed in a proper Man¬ ner before the Eye DYE. [See Fig. 2.] Suppofe both the Eyes were directed to an Objedt at a very great Diftance, the Axes of the Eyes XX and YY, which are rigot Lines paffing through the middle Points of the Pupil in each, wiil be parallel to each other as to Senfe, and in this Cafe the Axes of the EJyes would coincide with the Axes of the Glaffes, and it is in this Cafe only, viz. when the Axes of the Eye and Glafs are the fame, that diftindt Vifion can be made, or the Image of an Objedt be completely formed in the Eye. Now were the Glafles deligned to view very diftant Ob- jedts, the Glafs G H would be in a proper Pe¬ tition. .But this is not the Defign of Spedtacle- Glafles, and it is impoffible to ufe them for any fuch Purpofes, at leaft not one in a Thou- Jand can ufe them to view diftant Objedts y and therefore the Abfurdity of the Pofition of GlaJJes in common SpeSiacles is fufficiently evi¬ dent. ; For in the next Place, fince only nigh Ob- jedfai
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