IN OPTICS. 77 



of its picture upon the retina, holds a certain 

 correspondence with the change of the eye's 

 position. Let us now suppose the eye to be in 

 motion, while, from some disorder in the system 

 of sensation, we are either without those feel- 

 ings, which indicate the various positions of the 

 eye, or are not able to attend to them. It is 

 evident, that, in such a state of things, an 

 object at rest must appear to be in motion, 

 since it sends in succeeding times its picture to 

 different parts of the retina. And this seems 

 to be what happens in giddiness. I was first 

 led to think so from observing, that, during a 

 slight fit of giddiness 1 was accidentally seized 

 with, a coloured spot, occasioned by looking 

 steadily at a luminous body, and upon which I 

 happened at that moment to be making an ex- 

 periment, was moved in a manner altogether 

 independent of the positions I conceived my 

 eyes to possess. To determine this point, I 

 again produced the spot, by looking some time 

 at the ftame of a candle ; then turning myself 

 round till I became giddy, I suddenly discon- 

 tinued this mption, and directed my eyes to the 

 middle of a sheet of paper, fixed upon the wall 

 of my chamber. The spot now appeared upon 

 the paper, but only for a moment 5 for it im- 

 mediately after seemed to move to one side, 

 and the paper to the other, notwithstanding I 



