SPECTACLES. 



more than is juft fufficient either for reading, or for other 

 necefTary purpofes. 



When a variety of fpeftacles cannot aftually be tried, 

 the defeft of the fight may be exprefled by mentioning the 

 diftance from which the perfon can read, or other pecu- 

 liarities, from which the neceflary glaffes may be deter- 

 mined pretty nearly. For an account of the inftrumcnt 

 for meafuring the exadt limits of diftinft vifion, invented 

 by Dr. Porterfield, and improved by Dr. T. Young, fee 

 Optometer. 



Huygens conceived, that the glafles of fpeftacles, inftead 

 of being equally curved on both fides, (hould have the curva- 

 tures of their oppofite furfaces in the proportion of 6 to i, 

 and the moft convex fide turned towards the objeft, becaufe 

 he had demonllrated that fuch a form was bell luited to the 

 objeft-glalTes of telefcopes, as having lefs aberration than 

 any other form of lens. 



Dr. Wollallon obferves, that although it may be true that 

 fuch a form of glafs was bell calculated for the objeft-glafs 

 of a telefcope, previoufly to the celebrated difcovery of the 

 achromatic objedt-glaf<! by the late Mr. DoUond, yet what- 

 ever advantages might at any time be expetled from the 

 telefcopic objeft-glafs fo (haped, thefe were not to be ob- 

 tained by a fimilar conltruftion in fpeftaclcs, as may eafily 

 be feen by confidering the different ufos of the rcfpcftive 

 iiiftruments. 



In a telefcope, in the firrt; place, our view is nccellarily con- 

 fined to a very fmall diftance on each fide of the axis ; and, 

 iecondly, every part of the objedl-glafs contributes to the 

 diftinftnefs of any objeft viewed. 



It is under thefe circumttances alone, that the proportion 

 of the curvatures above-mentioned might be proper for a 

 fingle objedl-glafs, as beincj capable of colledling into the 

 fame focus the rays that fall on every part of it parallel to 

 the axis. But by fpedlacles, on the contrary, objedls are to 

 be viewed, if poffible, in every diredlion in which they might 

 be feen by the naked eye, which is often removed from the 

 centres of the glafles ; confequently, a conllrudlion that is 

 calculated to reprefent corredlly central objccls alone cannot 

 be the moft advantageous. 



In fpedlacles alfo the portion of the glafs employed at 

 once is fcarcely larger than the pupil of the eye ; fo that 

 any endeavour to procure the concurrence of all parts of a 

 glafs in any one effedl is evidently fuperfluous, and may alfo 

 be (hewn to be prejudicial. 



Mr. B. Martin, fo well known by his numerous writings 

 upon philofophical and mathematical fubjedls, made fpedla- 

 cles fixty years ago, which he called vifual glafles, and in 

 which the two glades were not fituated in the fame plane, 

 or with their axes parallel to each other, but were placed 

 inclined to each other, in the fame angle as the pencils of 

 rays muft proceed from the objedls which are viewed, and 

 come to the two eyes. Refpedling the common glaffes, this 

 writer obferves, in his trcatifc upon fpedlacles, that he con- 

 fidered the two glades of a common pair of fpedlacles, when 

 placed both in the fame plane, and with their axes parallel to 

 each other, as being moll adapted to view objedls at a very 

 great diftance, where wc can fee no objedls with them at 

 all ; for, when we ufe fpedlacles, the axes of both eyes are 

 always turned to the objedl which we view, and meet in a 

 point in it, as in reading, writing, working with a needle, 

 &C. ; confequently the axes of the eyes, and of the glades, 

 are fo far from coinciding, that they make a confiderable 

 angle with each other, by which means the eye is deprived 

 of the principal pencil ef rays, viz. thofe rays which be- 

 long to the axes of each of the glades, and which will 



Vol. XXXIII. 



make the moft perfeft part of the image on the retina of 

 the eye, and which rays alone can produce perfedl vifion. 

 Thofe rays alone can enter the eye from the objedl we 

 look at, which come on one fide of the glafs, or obliquely 

 to its axis, and are therefore regularly refrafted to the eye, 

 fo as to make the vifion in proportion imperfedl. 



As the adlion of light upon the eye tends gradually to 

 weaken it, no more of fuch an adlive principle (hould be 

 admitted into fo delicate and fine a llrudlure as that of the 

 eye, than what is quite neceflary to illuminate the objedl, 

 and make it fufficiently viCble. But the common fize of 

 fpedlacle-glaffes pours in upon the eye-balls three times as 

 much as is neceflary for this purpofe, and, therefore, it 

 very prejudicial to the eyes in that refpedl, for in time it 

 makes them weak and watery. Furtlier, it is well known 

 that only a particular quantity of light is proper for perfedl 

 and diftindl vifion ; and that a greater or lefs degree of it 

 always impairs and confufes the image, and confequently 

 the vifion of the objedl. For this purpofe, in microfcopes, 

 telefcopes, and other optical inftrumenti, provifions are 

 made tn regulate the quantity of light by proper apcrturen 

 and diaphragms ; but in fpedlacles, where the quantity of 

 light diould be in the higheft, degree regulated and adjufted, 

 no provifion at all has been made, but the tender fyltem of 

 the eye has been left defencelefs from the fuperfluous and 

 injurious particles of light. 



In common fpedlacles, only thofe rays of light which fall 

 near the axis of tlie glafles, can be regularly refradled to the 

 eye, and confequently only fuch can make the image, or 

 vifion of the objedl, in any degree perfedl : and, therefore, 

 confidering the unreafonable area or diameter of the glafles 

 of common fpedlacles, there muft neceffarily be a great 

 quantity of light that will not only annoy the image by 

 fuffufion, but greatly affcdl the eye, and render the image 

 imperfedl by an irregular refradlion from all the extreme 

 parts of the glafs. 



The lenfes of Mr. Martin's fpedlacles were fo fixed in 

 their frame, that the axes of the two glafles were inclined 

 to each other, and met in one point, fituated at the fame 

 diftance from the glafles as the focus of each glafs refpec- 

 tively ; or, in other words, the foci of the two glaffes were 

 brought into one common point : whereas in the common 

 fpedlacles, the axes of the two glades being parallel, the 

 two foci are at the fame diftance apart as the centres of the 

 glaffes are. Mr. Martin alfo recommended opticians to 

 make the glaffes fmaller ; an alteration which was attended 

 with confiderable advantages : for, before his time, the lenfes 

 were frequently made an inch and a half in diameter. 



Dr. Franklin ufed a pair of double or bifedled fpedlacles, 

 which he thus defcribes in a letter to one of his friends. 

 " The fame convexity of glafs, through which a man fees 

 cleareft and beft; at the diftance proper for reading, is not 

 the beft for greater diftances. I, therefore, had formerly 

 two pairs of fpedlacles, which (hiftcd occafionally ; as in 

 travelling I fometimcs read, and often want to regard the 

 profpedls. Finding this change troublcfome, and not al- 

 ways fufficiently re.idy, I had the glaffes cut out, and half 

 of each kind aflbciatcd in the fame circle ; the leaft convex, 

 for viewing diftant objedls, in the upper half of the circle ; 

 and the moft convex, proper for reading, in the lower half 

 of the frame. By this means, as I wear my fpcdlaclet 

 conftantly, I have only to move my eyes up or down, as I 

 want to fee diftindlly far or near ; the proper glafles being 

 always ready. Although I cannot diftinguifh a letter, even 

 of large print, by the naked eye, with the affiftance of this 

 invention, my eyes arc as ufeful to me as ever they were ; 



3P and 



