Mr. J. C. Douglas on an Optical Experiment. 45 



cipal laws of linear perspective — as the existence of vanishing 

 points, that they are in a line drawn through the centre of visual 

 direction parallel with the vanishing lines, that vision with one 

 eye only can be considered in perspective, &c; the coincidence 

 of the centre of visual direction and the centre of motion of the 

 eyeball is proved by the immobility of the radiating lines when 

 the eyeball only is moved, and their movement when the head is 

 moved with relation to the mirror ; a means is afforded of finding 

 a point on a looking-glass, a perpendicular to the surface drawn 

 through which would pass through the centre of visual direction ; 

 a principle is afforded on which an instrument may be con- 

 structed for measuring the distance between the centres of motion 

 of the eyes. The instrument I used for this purpose consisted of 

 a card divided along one edge by lines drawn on both sides of 

 it, and as nearly as possible coincident ; this card being held in 

 front of a mirror and viewed obliquely, so that the divisions on 

 the card and on its image might be seen at the same time (the 

 card being held with the divided edge horizontal), two lines on 

 the card were seen to coincide with the images of two lines cor- 

 responding to them on the other side of the card ; and the dis- 

 tance between the two lines so coinciding with reflected lines 

 being read off on the card, gave the distance between the centres 

 of motion of the eyeballs. This instrument admits of great im- 

 provement; and it will be evident that its delicacy may be increased 

 by increasing the proportion the distance between the image and 

 object bears to the distance of the mirror from the eyes. It has 

 been stated that, if no apparent motion could be produced in a 

 distant object by closing one eye after having covered that object 

 by another nearer the eye, the eye so closed must be inoperative 

 when both eyes are used, the greater perfection of the one eye ob- 

 scuring altogether the effect of the weaker organ. And so far as 

 apparent direction is concerned the experiment seems conclusive. 

 But this view appears to need modification ; for in a case in which 

 both systems of lines were distinctly visible simultaneously, pro- 

 ving both eyes were employed, no apparent motion could be 

 produced by closing the left eye ; and although the left system 

 of lines was fainter than the right, and the left eye was undoubt- 

 edly weaker than the right, the fact that the system appertaining 

 to the left eye was distinctly visible when both eyes were used 

 simultaneously proves that, in regarding the mirror, both eyes 

 were effective. 



It appears, therefore, that the powdered mirror affords a deli- 

 cate means of detecting the share each eye has in vision, of ex- 

 posing their inequalities of strength, and of detecting if both are 

 actually effective when used together — in the last, being more 

 delicate than the apparent-motion test. 



