New Analysis of Solar Light. 407 



of the retina than those affected by the regularly refracted light. 

 To these belong undoubtedly the diffraction of light in the pupil. 

 When light passes through a narrow orifice, or simply passes the 

 edge of a dark body, a small portion of it aWU be always deflected. 

 Now although the pupil is certainly too large in comparison to 

 the focal distance of the eye to permit of rings being formed and 

 a considerable portion of light dispersed, as is the case when a 

 very small aperture is held close to the eye, still the diffraction 

 is by no means completely annulled. Further, it may be regarded 

 as questionable whether the media of the eye are absolutely clear ; 

 being partly composed of microscopic cells and fibres, as the 

 cornea and crystalline lens, and in other places traversed by a 

 great number of fine membranes, as in the vitreous humour. 

 The presence of little irregularities in the structure of the back- 

 ward portion of the vitreous humour is further indicated by the 

 so-called midges of the field of view, and perhaps something 

 similar is to be found in other portions of the eye. By these also 

 hght must be dispersed. Finally, it is proved by the eye-miiTor 

 constructed by me*, that a tolerable quantity of light is sent 

 from the illuminated portions of the retina to the pupil, and this 

 must be reflected back again from the forward surface of the 

 cornea. It is therefore to my mind an undoubted fact, that a 

 portion of the light incident upon the eye is deflected so as to 

 fall upon other portions of the retina. Whether along with this 

 an extension of the nervous excitation over the retina takes place, 

 cannot be decided without further investigation ; for our purpose 

 it is, however, a matter of indifference whether objective light, or 

 only its subjective perception, is diffused over the retina. 



I will now attempt to prove that one of Brewster's most striking 

 results is derived from a mixing of the regularly refracted light 

 with other light which has been dispersed partly without and 

 partly within the eye. I allude to the isolation of white light in 

 the yellow of the spectrum by glass coloui'ed blue with smalt. 

 It is known that by such glass dark bands are generated in the 

 less refracted portion of the spectrum. Between them stand 

 several coloured bands, namely (1) the extreme red, embracing 

 the lines A and B, quite unaltered ; (2) a band of reddish-orange 

 between tlie lines C and D, extremely weak ; (3) a yellow band, 

 at one end verging into orange, at the other end into green, less 

 weakened than the foregoing. Between this yellow and the 

 green occurs an interval not totally dark, while blue and violet 

 are transmitted without diminution. Brc\\ster draws attention 

 to the fact, that while tlie primitive colour of the yellow band 

 was a rich gamboge, the same viewed through a certain thicknesp 



* See my deseriiition of an eye-minor (Augenspiegel) for the investiga 

 tion of the retina of Uving eyes. Berhn, 1851. 



