C74 fVollaston's Perlscopic Spectacles. 



copic glasses mounted in a single steel frame, which coSt 

 10s. QAi The glasses, I must observe, are very different to 

 his proposed form, having of each, the inner side, or that 

 next to the eye, so little incurvated, that by any person but 

 an optician they would be called plano-convexes. The 

 focus is 4 inches, the same as used by Dr. Wollaston in his 

 experiment. In a similar mounting, with double convex 

 glasses of the same diameter and focus, I provided a pair of 

 our own manufacture, and as sold by us at 3s. 6d. These 

 two pairs of spectacles were attentively compared together by 

 myself and several judicious and impartial persons, in the 

 manner as stated by Dr. Wollaston of his; the result was 

 as follows: — ^The convex glasses being applied as close as 

 possible to the eyes, the print of a large quarto page was 

 viewed through them at a distance for distinct vision at their 

 centres : the letters, at the distance of about 25 lines, appear- 

 ed quite distinct, or well defined. Giving the axes of the 

 eyes a little obliquity, to discriminate more lines, an indi- 

 stinctness or confusion of letters commenced, increasing to- 

 wards the extremity of sight; and from the lateral aber- 

 ration of the lenses the letters were tinged with the prisma- 

 tic colours. Keeping the head fixed in the sanve position, 

 the periscopic glasses were substituted. The extent of di- 

 stinct letters wuhout distortion was nearly as great, but the 

 coloured letters were evidently nearer to the centre, and 

 more numerous than by the other glasses. By inclining 

 the axes of the eyes still more than in the former case, or 

 looking extremely asquint through the glasses, a greater 

 -extent of lines was observed, but blended vi'ith colour and 

 confusion. The optic nerves felt a sensible irritation, 

 evidently from the squinting position of the eyes, a refrac- 

 tion of many superfluous rays, and the consequent increased 

 and unusual maonitude of the images on the retina. The 

 pain m the eyes mentioned by Dr. Wollaston must have 

 .arisen only fiom this circumstance, and not from the one 

 he represented it to be. By a trial of the old meniscus I 

 before mentioned, which is four inches focus, and corre- 

 sponds with what he has a patent for, in comparison with 

 the abcve plano-convexes, the view of the letters vias still 

 more extended, but illegible and with nuach colour, and like 

 -the other, towards the extremity, of no sort of use for the 

 purposes of vision. 



Now all this is conformable to the laws of optics, and 

 manifests a property different to that aAlvanced by Dr. Wol- 

 laston. 

 . .. " Thesci 



