ON TirU MECHANISM OP THE EYE, 



587 



and Mr. Kainsdcn to abandon, in great 

 measure, the opinion which suggested them, 

 and to suppose, that a change of the cornea 

 produces only one third of the effect. Dr. 

 Olbers, of Bremen, who in the year 1 780 pub- 

 lished a most elaborate dissertation on the 

 internal changes of the eye*, which he 

 lately presented to the Royal Society, had 

 been equally unsuccessful in his attempts to 

 measure this change of the cornea, at the 

 same time that his opinion was in favour of 

 its existence. 



Room was however still left for a repeti- 

 tion of the experiments; and I began with 

 an apparatus nearly resembling that which 

 Mr. Home has described. I had an excel- 

 lent achromatic microscope, made by Mr. 

 Ramsden for my friend Mr- John Ellis, of 

 five inches focal length, magnifying about 

 20 times. To this I adapted a cancellated 

 micrometer, in the focus of the eye not em- 

 ployed in looking through the microscope ; 

 it was a large card, divided by horizontal and 

 vertical lines into fortieths of an inch. When 

 the image in the microscope was compared 

 with this scale, care was taken to place the 

 head of the observer so that the relative motion 

 of the image on the micrometer, caused by the 

 unsteadiness of the optic axes, should always 

 be in the direction of the horizontal lines, 

 and that there could be no error from this 

 motion, in the dimensions of the image taken 

 vertically. I placed two candles so as to e.v- 

 hibit images in a vertical position in the eye 

 of Mr. Konig, who had the goodness to as- 

 sist me; and, having brought them into the 

 field of the microscope, where they occupied 

 35 of the small divisions, I desired him to fix 

 his eye on objects at different distances in 

 the same direction : but I could not perceive 



* De Oculi Miitationibus intemis, 4. Gotting. 1780. 



the least variation in the distance of the 

 images. 



Finding a considerable difficulty in a pro- 

 per adjustment of the microscope, and being 

 able to depend on my naked eye in measur- 

 ing distances, without an error of one 500th of 

 an inch, I determined to make a similar ex- 

 periment without any magnifying power. I 

 constructed a divided eye glass of two por- 

 tions of a lens, so small, that they passed be- 

 tween two images reflected from my own 

 eye : and, looking in a glass, I brought the 

 apparent places of the imai^es to coincide, 

 and then made the change requisite for view- 

 ing nearer objects ; but the images still coin- 

 cided. Neither could I observe any change 

 in the images reflected fiom the other eye, 

 where they could be viewed with greater con- 

 venience, as they did not interfere with the 

 eye glass. But, not being at that time 

 aware of the perfect sympathy of my eyes, 

 I thought it most certain to confine my ob- 

 servation to the one with which I saw. I must 

 remark that, by a little habit, I have acquired 

 a very ready command over the accommoda-* 

 tion of my eye, so as to be able to view an 

 object with attention, without adjusting my 

 eye to its distance. 



I also stretched two threads, a little in- 

 clined to each other, across a ring, and di- 

 vided them, by spots of ink, into equal spaces, 

 I then fixed the ring, applied my eye close 

 behind.it, and placed two candles in proper 

 situations before me, and a third on one side, 

 to illuminate the threads. Then, setting a 

 small looking glass, first at four inches dis- 

 tance, and next at two, I looked at the 

 images reflected in it, and observed at what 

 part of the threads they exactly reached 

 across in each case ; and with the same result 

 as before. 



