SR ee ee ee eee Oe re 
_ Only conditions necessary for a satisfac- 
__ tory result are that none of the leading 
Be! 
W. B. Rogers on Binocular Vision. 87 
(1.) A vertical line with a rhombus (fig. 57). In this case 
the small converging movement measured by the breadth of the 
rhombus suffices to combine the vertical line successively with 
the two near and the two remote sides of the figure, and to de- 
velop as a resultant the form of a square in steep perspective, 
As the slight vibration which attends our ordinary efforts at fix- 
ing the eyes is quite sufficient for this, the apparent coincidence 
and the consequent relief are perfect, and nothing short of a 
continued and even painful direction of the view to one end of 
the horizontal diameter enables us even partially to separate the 
components, 
vin any of the preceding combinations. 
__, Inthe above and a multitude of other 
— forms which might be suggested the 
Parts of the complex line or system of lines shall be so much in- 
clined to the vertical as not readily to unite with the correspond- 
_ = ing part of that line, and that the entire horizontal breadth of 
the figure shall be small enough to allow all its parts to be united 
With the vertical in an insensibly short time. ; 
t will be remarked that in the preceding figures cach linear 
element makes so small an angle with the vertical as to be capa- 
ble of uniting with it. But as indicated above this is not essen- 
