of the Eye in t'elatlon to the Percej)tton of Distance. 57 



and violet rays, it is very difficult with any focus to observe 

 the tinted margins. 



30. Moreover the apparent achromatism of the experiment 

 above mentioned is only found with pencils nearly coincident 

 with the axis of the eye. Oblique pencils always give tinted 

 margins in white light. This may be verified by looking at a 

 set of concentric circles in white and black. If the eye be 

 steadily focused for the white centre, no colours are seen at 

 its margin, but may be observed at the inner and outer edges 

 of the other circles. I have always found it possible to get 

 yellow exterior margins to a white object at any distance ; 

 that is, I have always found it possible to focus for a further 

 distance ; but not vice versa. 



31. If a square of white paper be carefully tinted around 

 the edge with blue, and laid on a black ground, it appears 

 further off than a perfectly white square of the same size 

 beside it, since the blue margin helps to correct the yellow 

 rays that surround the image at a longer focus. In short, a 

 blue-edged square appears like a white square looked at with 

 too near a focus. The opposite effect is observed with a 

 margin tinted yellow. 



32. I adjusted a lenticular stereoscope to easy focus, then 

 placed in it a sheet of white paper having two equal circles 

 1 centim. in diameter, described upon it at the appropriate 

 distance — one red, the other blue. The joint effect of these 

 two gave, apparently, a combination of a red circle with a 

 slightly larger one of blue. On drawing a slightly smaller 

 circle of blue, with the circle of red as before, they appeared 

 to give exact coincidence. 



33. Knowing the power of the eye to reconcile retinal 

 images in magnitude (see § 16), I placed in the stereoscope a 

 penny coloured blue and a halfpenny coloured red. They 

 refused to be seen as one object. On replacing them by a 

 red penny and a blue halfpenny, coincidence was^ with some 

 difficulty, obtained. 



34. My next experiments attempted the direct estimation 

 of the distance of objects by the focus of the eye apart from 

 other means of estimation. A conical tube of metal, blackened 

 interiorly, was taken, through which only a limited field of 

 view was possible, the eye being placed at the smaller aperture. 

 In front of this tube a screen of blackened cardboard was 

 placed, capable of adjustment to any distance from 10 centims. 

 to 350 centims. from the tube, and covered the entire field of 

 view. Coloured glasses could be introduced in front of the 

 tube. The objects displayed upon the cardboard screen in the 

 centre of the field of vision were irregularly shaped pieces of 



