120 



the impression, yielded (comparatively and roughly taken) the fol- 

 lowing series, crimson-pink, purple-pink, purple, blue, green, the 

 latter being the produce of the highest intensity tried. 



As in the foregoing researches, the relative degrees of light were 

 but broadly assumed, whilst the comparative experiments comprised 

 a variety of differences affecting the photochromatic results, 

 another series of experiments on the simple effects of degrees of 

 light was instituted, in which all these other differences were elimi- 

 nated. In this series the quantity of light was varied by partial or 

 sectional screens of glass, or other transparent or semitransparent 

 substances. The results were particularly satisfactory, different 

 tints or shades of colour being obtained by the same view and in the 

 same spectrum of a window-aperture, when different thicknesses of 

 window glass were placed in the several sections (six in number) into 

 which the aperture was divided. 



A beautiful example of the chromatic effects of partial and varied 

 screening of light on the optical spectrum elicited, was incidentally 

 obtained by viewing an aperture in the clouds, when the sky was 

 otherwise densely covered. After gazing for a few seconds on the 

 middle of this aperture, the spectrum, as viewed with gently closed 

 eyes, exhibited a singular variety of the richest tints according to the 

 differences in the light screened off by the edges of the cloud and 

 by certain little patches within the aperture. The spectrum resem- 

 bled the variegation and richness of colouring as elicited in certain 

 transparent or semitransparent substances when examined by polar- 

 ized light. 



The experiments on binocular and multiple spectra, as described 

 in Part I. of the author's paper, being repeated under degrees of 

 light adequate for yielding colour, gave pictures, in many cases, of 

 much interest and beauty. The multiple spectra, however, which 

 proved the most strikingly beautiful, were derived from the sun, 

 which was viewed indirectly, and on occasions, near setting, in winter, 

 when the intensity of its light was duly subdued by passing through 

 a dense condition of atmosphere. Under such circumstances, images, 

 sometimes in 100 to 150 repetitions, were impressed on the retina 

 by rapid glances at the sky immediately around the sun. These were 

 taken by quick movements of the head, winking intermediately, at 

 the rate of 60 to 1 20 impressions in the minute ; and the result, 



