On Vision. 127 



The only difference between this experiment and those men- 

 tioned in my former paper, made with the same instrument, 

 consists in the brightness of the ohject: the principle is the same. 



In the next paragraph he savs : " One simple argument 

 is sufficient to set aside these opinions : it is founded on an 

 experiment well known to all those who have properly stu- 

 died the subject. If we look through two minute holes, 

 much nearer together than the diameter of the pupil in its 

 most contracted state, at one of two points, nearly in the 

 same line, and within the limits of perfect vision, the other 

 point will always appear double, whether we fix on the 

 nearer or the more remote for the object of our attention. 

 Here there is no change of the aperture, but a true altera- 

 tion in the refractive poweis of the eye." 



This experiment is erroneous, and can only mislead those 

 who may not be inclined to try it ; for when we look through 

 two minute holes, made in the manner described above, two 

 circles of light are seen intersecting each other as represent- 

 ed in Fig. 9, Plate IV. 



Now when a point is seen through a or b it will appear 

 single ; but if it be seen through the space c, it will appear 

 double ; because an image is formed of it by each hole ; and 

 if two points be viewed through c they will both appear 

 double : but to see one of the points single and the other 

 double, one must be seen through a orb, and (he other throuo^h 

 the space c. This experiment, which is the only one advanced 

 " to set aside" my theory, proves nothing more than that 

 the reviewer looked through two minute holes. 



If there were any alteration in the refractive powers of the 

 eye, we might then see one point single and the other 

 double, when viewed through one minute hole; but this 

 is contrary to experience, and consequently proves the ab- 

 surdity of the reviewer's suppos/tiou. 



As this critic does not appear to understand his own ex- 

 periment, let us sec, in the next place, whether he under- 

 stands mine. 



*' It is obvious," says this writer, '/ that Mr. Walker's 

 three experiments with the lens prove a great deal too much: 

 they demonstrate that a contraction of the aperture makes a 



remote 



