£ 4G1 ] 



LXXIX. On a Contrivance to help defective Vision. By 

 Joseph Skinmer, Esq. 



To Mr. Tilloch. 



Sir —Permit me to enter into a brief detail of -a contrivance 

 I feU on some years ago to help defective vision. I reached 

 Malta in the sunimer of 1808, and was not long ni expenencmg 

 the rapid decay of an organ already impaired, through the effect 

 of the powerful glare of light reflected there by the white sur- 

 faces which continually meet the view, the houses and mclosures 

 beino- of stone, and the surface of the island in general rocky, 

 >vith patches of green, chiefly seen from the eminences, inter- 

 spersed at intervals to cheer the sight. After a lapse ot two years 

 xn\ vision became so obscured, that I could not recognise an in- 

 dividual acquaintance at a short distance ; l)ut by looking through 

 a fine aperture made in a very thin metallic substance, and held 

 close to the eye, I found the surrounding objects so completely 

 defined that I could see them distinctly when looking towards 

 each extremity of the street in which I dwelt. This contrivance, 

 by the admission of a small portion only of direct rays, without 

 the interposition of any oblique rays of light, allows the object 

 to be seen at any degree of proximity, or of distance proportioned 

 to its magnitude ; insomuch that when an object, very fine print 

 for instance, is brought close to the eye, it is powerfully magni- 

 fied, agreeably to a well-known law of optics ; and being di^ 

 stinctly seen at a remote distance, it follows that an indefinite 

 focus is obtained, always relative to the size of the object vie^ved. 

 Whenever I had recourse to this expedient, I constantly touiid, 

 on withdrawing the instrument, that I could distinguish objects 

 with more precision than before; and, as I conceive for tms 

 reason, that having been looking for some time through a clies- 

 t^>ned medium of light, I was better enabled to meet the broad 

 jrlare than I should have been if constantly exposed to its action. 

 In the employment of convex spectacles, or magnifiers, the re- 

 verse haopens: the rays of light being then concentrated the 

 surrounding ol)jects, when they are withdrawn, become for a 

 time less distinct than before their application. In this view the 

 contrivance of metallic spectacles may be considered as a pre- 

 server of the sight, independently of the advantages I am about 

 to detail. Since my return to England, Mr. 1 homas Jones, 

 optician. No. 62, Charing Cross, has with great ingenuity con- 

 trived a pair of adjusting metaUic spectacles, by the means ot 

 which he measures the distance between tne pupils, so as to 

 adapt the instrument to any individual, and bring the two sights 



