406 M. H. Helmholtz on Sir David Brewster's 



coloured media beti^'een the source of light and the aperture, to 

 placing them between the prism and the eye. By this alteration 

 a considerable quantity of dispersed light wdl be excluded from 

 the field of view. 



The description of all these circumstances may a])pear pedantic, 

 and I am ready to admit that the irregularly refracted light 

 must certainly form an extremely small portion of the light inci- 

 dent — a portion far too inconsiderable sensil)ly to affect the 

 appearance of the spectrum under ordinary circumstances. It 

 Avill, however, be seen that it is not too small when added to 

 colours that have been already greatly weakened by absorption, 

 to cause a sensible change in the tint of the latter. 



The circumstances heretofore spoken of are such as possibly 

 might be excluded in following out Brewster's method of expe- 

 riment. Perhaps there are prisms which are able to withstand 

 the foregoing test ; Brewster's may perhaps have been properly 

 blackened, and the coloured media placed before the aperture ; 

 then indeed regularly refracted light alone would reach the eye. 

 But there are sources of error resident in the eye itself which 

 cannot be avoided. I would invite attention to the fact, that 

 when very bright light of any kind whatever falls upon a portion 

 of the retina, light of the same kind appears diffused as a weak 

 luminosity over a great portion of the field of view. The phseno- 

 menon is easy to be observed. Let a candle be placed in the 

 evening in the neighbourhood of a large dark surface, for instance 

 of a door which opens into a dark room, and let the degree of 

 darkness of the surface be observed while the light is alternately 

 concealed by the finger and allowed to strike the eye. It will 

 be readily seen, that as often as the rays freely enter the eye 

 a white luminosity appears spread over the surface, being brighter 

 in the vicinity of the light, and spreading itself weakly over the 

 more distant portions of the siu'face. The same is observed when 

 daylight, and most strikingly when direct sunlight enters the 

 eye from an orifice in a dark screen. When the orifice is covered 

 by a coloured glass, the luminosity has the colour of the latter. 

 I have observed this with my own eyes, which are in good con- 

 dition, and have also shown it to many others. That the diffrac- 

 tion of the light by the eyelashes is not the cause of this is 

 proved by the fact, that the phtenomenon is observed when the 

 lids are drawn far apart. 



With regard to the cause of this pha;nomenon, it has been 

 hitherto regarded by most observers who have noticed it as purely 

 subjective; it was believed to be refeirible to an extension of the 

 excitement to the adjacent fibres of the retina. But it can be 

 shown that circumstances exist which must cause a small portion 

 of objective light dis])ersed within the eye to reach other portions 



