54 Mr. S. P. Thompson on the Chromatic Aherratlon 



III. Bearing of Chromatic Aherraiion upon the 

 foregoing Data, 



21. Taking the chromatic aberration of the eye as an estab- 

 lished matter *, it is now time to inquire how far these various 

 considerations are affected by the fact that rays of different 

 colours have different focal length in the eye. 



22. Obviously all estimates of distance which depend on 

 retinal magnitude are liable to be influenced by colour. 

 This may easily be verified by taking two equal disks of 

 red and blue paper and placing them upon a black back- 

 ground. It will be found impossible to get a perfectly di- 

 stinct image of both at once, one or other being out of focus, 

 and therefore blurred at the edge and larger ; or if no dif- 

 ference in size appears, one (the blue one) will appear more 

 distant. The experiment may be varied by giving various 

 sizes and shapes to the coloured disks, but with the same 

 result, especially if the disks be looked at with one eye 

 only. Orange and green disks may answer the purpose, 

 but not so well as disks of red and blue, as these colours 

 are most widely separated on the spectrum f. 



23. On this account I am disposed to think that retinal 

 magnitude has little to do with the appreciation of distance, 

 except in the case of objects whose magnitude and tint are 

 familiar to us ; and I shall show other reasons for the opinion. 

 Hence I regard as very imperfectly true the statement of the 

 late Sir Charles Wheatstone, in the Bakerian Lecture for 

 1852 J, that " Convergence of the optic axes therefore sug- 

 gests fixed distance to the mind ; variation of retinal magnitude 



* In 1835 Brewster wrote : ^' 1 consider tlie 7ion-achromai{sm of the eye 

 as a fact as well established as any other fact in natural philosophy,' 

 Phil. Mag. 1835, p. 161. 



t Brewster called attention to similar phenomena produced by coloured 

 outlines in ' Brit.-Assoc. Report/ 1848, p. 48, " On the Vision of Distance 

 as given by Colour." He there gives a rather laboured explanation, com- 

 paring- the vision of difterent distances for the ditferently coloured lines to 

 the phenomena of the lenticular stereoscope. He adds that " the difference 

 of distance of the coloured lines or spaces may be appreciated even with 

 one eye." Compare also Helmholtz, Physiologische Optik, p. G45. 



Compare also an experiment with a disk of black paper on a background 

 of pale ultramarine, by Prof. O. N. Rood, ' Silliman's .Journ.' xxxi. 1861, 

 pp. 343, 344. 



There is little doubt that Wheatstone's phenomenon of the '^Fluttering 

 Hearts" (see 'Brit.-Assoc. Rep.' 1844) is due to the attempt of the eye 

 to focus for adjacent spaces of colours of unequal refrangibility, which 

 could not, therefore, be in distinct focus at one time. The same peculiar 

 fluttering is observed when white objects on a dark groimd are observed 

 through a purple solution of permanganate of potash. 



X Phil. Trans. 1852, p. 4. 



