Fie. 3. 



Mr. S. Rowley on a New Theory of Vision, 199 



easy, some special instrument must be provided. A piece of 

 pine board whose opposite great surfaces are rectangles parallel 

 to one another, 13 inches long and 8 inches wide, with five pins, 

 furnishes the means for obtaining accurate results where the 

 distance between the near and far points does not exceed 12 

 inches*. 



To use this contrivance (supposing the distance from the 

 middle of the line between the centres of the eyes to the near ob- 

 jective point to be 6 inches, and from the near to the far ob- 

 jective point 12 inches), in the surface SRFC (fig. 3), \ an 

 inch from the short side S R, and 5 inches from the long side 

 R F, take the point A. Through this point and at right angles 

 to the side S R draw a line NO. In 

 this line, 12 inches from the point 

 A, take the point 13. Through this 

 point, and at right angles to this line, 

 draw another line PL. In this latter 

 line take the two points T, T', each dis- 

 tant 2^(7 inches from the point B, and 

 4f S Q inches from the other ; and finally 

 4^ inches from the point B take the 

 point D. At each of the points so 

 taken set a pin. Now place the board 

 in a horizontal position a little below 

 and parallel to the plane of the axes, 

 with the pin at A 6 inches from the 

 middle of the line joining the centres 

 of the eyes, and, together with the pin 

 at 13, in the vertical plane which bisects 



this line at right angles ; w T hich disposition of the pins is had 

 when, looking at the images of the pin at A, the left eye's image 

 of the pin at T' and the right eye's image of that at T are found 

 lying in the axes. 



When now the axes are fixed upon the images of the pin at A, 

 the right eye's image of the pin at B will be seen in the same 

 line of no variation as the left eye's of the one at D, and at a 

 point a little beyond where the latter is seen ; and when they are 

 fixed upon the images of the pin at B, the left eye's image of that 

 at A will be seen in a position a little before where the right eye's 

 image of a pin, set nearly \-£ Q inch directly to the right of A, 

 will be seen, the distance between these being sensibly less than 

 that between the images received from B and D when the axes 

 were converged toward A. 



* Another board of the same width and 18 inches long, serving as a 

 support to this, to be held with one end against the nose by the hands 

 pressing both pieces at the sides, will be found serviceable for maintaining 

 steadiness of position. 



