112 EXPERIMENTS, &c. 



axes to each other remain the same. For 

 I find, that, when a luminous point, to which 

 both axes are turned, is distinctly seen by my 

 left eye, I can, by certain efforts not easily to 

 be described, but without changing the position 

 of either axis, make it afterward appear as a 

 surface, and this too, at one time, from the rays 

 coming to a focus too soon, and at another, too 

 late, for perfect vision*. One instance of these 

 variations deserves to be minutely described, as 

 it proves, that the refractive power of the eyes 

 is subject to greater changes, than what are 

 shown by any experiments I have met with in 

 authors. When I look attentively at a bright 

 star, with the optic axes parallel to each other, 

 it appears to my left eye a surface of some ex- 

 tent, and to my right eye, though not a point, 

 yet a surface of very small extent, as small as- 



* The variations, however, seem produced in such a man- 

 ner, that the middle of the set belonging to one degree of 

 the mutual inclination of the optic axes, is always differen t 

 from the middle of the set belonging to another degree of 

 their inclination ; and that, when no other effort is made, 

 than to direct both axes to the same object, the eyes always 

 assume the middle state of the refractive power, which ac- 

 companies that particular inclination of the axes. No argu- 

 ment, therefore, can hence be derived against the applications 

 I formerly made of the general fact, respecting the connexion 

 of the refractive states of the eyes with the mutual inclina- 

 tions of the optic axes. 



