ig6 • Qu the Alechan'ifm of thi Eye. 



•nearer: while one axis moves, it is not eufy to keep the other perfe£lly at reftj and It is 

 not imponible, tliat a change in the proportions of fome eyes, may render a flight alteration 

 of the pofition of the axis abfolutely pecefTary. TheCe cpnfideratipns may partly explain 

 •the trifling difFerencc in the place of the cornea that was pbferved in 1794.. ., ^' appears that 

 the experiments of 1 795 were made with confiderable accuracy, and no doubt with excellent 

 inllrumentsj and their failing to afcertaln the exiftenceaf any cliange, induced Mr. Home 

 jind Mr. Ramfden to abandon, in great meafurc, the qpinipn which fuggefted them, and to 

 fuppofe, that a change of the cornea produces ohly one. third of the efFe£l. Dr. Olbets of 

 Prenien, who ip the year 1780 publifhed a mod elaborate diflertation on the internal 

 fhanges of the eje, * which he lately prefented to the Royal Society, had been equally un- 

 fjiccefsful in bis att^n^pts to meafure this change of the .pornea, at the fame time that his 

 opiniori was in favour of its exiftence. 



Room was however ftill left for a repetition of the experiments ; and I began with an 

 apparatus nearly refembling that which Mr. Home has defcribed. I had an excellent 

 achromatic micrafcope, made by Mr. Ramfden for my. friend Mr. John Ellis, of five inches 

 focal length, magnifying about 20 times. To this L adapted a cancellated micrometer, in 

 the focus of the eye not employed in looking through the microfcope : it was a large card, 

 divided by horizontal and vertical lines into fortieths of an inch. When the image in 

 the microfcope. was compared with this fcale, care was taken to place the head fo that 

 the relative motion of the images on the micrometer, caufcd by the unfteadinefs of the 

 cjptic axisj (hould. always be in the direflion of the horizontal lines, and that there could 

 bene error, from this motion, in the dimenfions of the image takfn vertically. I placed 

 two candles fo as to exhibit images in a vertical pofition in the eye of Mr. Konig, who had 

 the goodnefs to afTift me ; and, having brought them into the field of the microfcope, 

 where they occupied 35 of the fmall divifions, I defired him to fix his eye on objects at 

 different diflanc;es in the fame direftion ; but I could not perceive the leafl variation in the 

 diftance of the images. 



Finding 3 confiderable difficulty in a proper adjuflment of the microfcope, and being able 

 to depend on my naked eye in meafuring diflances, without an error of one 500th of an 

 inch, I determined to make a fimilar experiment without any magnifying power. I con- 

 ftru£led a divided eye-glafs of two portions of a lens, fo fmall, that they pafTed between 

 two images reflefted from my own eye ; and, looking in a glafs, I brought the apparent 

 places of the images to coincide, and then made the change for viewing nearer obje£ts; but 

 the images ftill coincided. Neither coi^ld I obferve any change in the images reflefted from 

 the other eye, where they could be viewed with greater convenience, as they did not inter- 

 fere with the eye-glafs. But, not being at that time aware of the perfeft fympathy of the 

 eyes, I thought it mofl certain to confine my obfervation to the one with which I faw. I 



* De Oculi Mutationib«s internis. Getting. nSO, 4f. 

 ■J muft 



