TRANSACTIONS OF THE SECTIONS. 21 
the light of the clouds viewed without a coloured glass. A deep blue glass gave 
brushes of remarkable intensity, notwithstanding the large quantity of light absorbed. 
With the green and blue glasses, the brushes were not coloured, but simply darker 
than the rest of the field. 
To examine still further the office of the different colours in producing the brushes 
seen with ordinary daylight, the author used a telescope and prism mounted for 
showing the fixed lines of the spectrum. The sun’s light having been introduced 
into a darkened room through a narrow slit, it was easy, by throwing the eye-piece 
a little out of focus, to form a pure spectrum on a screen of white paper, placed a 
foot or two in front of the eye-piece. On examining this spectrum with a Nicol’s 
prism, which was suddenly turned round from time to time through about a right 
angle, the author found that the red and yellow did not present the least trace of 
brushes. The brushes began to be visible in the green, about the fixed line E of 
Fraunhofer. They became more distinct on passing into the blue, and were parti- 
cularly strong about the line F. The author was able to trace them about as far as 
the line G; and when they were no longer visible, the cause appeared to be merely 
the feebleness of the light, not the incapacity of the greater part of the violet to 
produce them. With homogeneous light, the brushes, when they were formed at 
all, were simply darker than the rest of the field, and, as might have been expected, 
did not appear of a different tint. In the blue, where the brushes were most distinct, 
it appeared to the author that they were somewhat shorter than usual. The con- 
- trast between the more and less refrangible portions of the spectrum, in regard to 
their capability of producing brushes, was most striking,- The most brilliant part 
of the spectrum gave no brushes; and the intensity of the orange and more refran- 
gible portion of the red, where not the slightest trace of brushes was discoverable, 
was much greater than that of the more refrangible portion of the blue, where the 
brushes were formed with great distinctness, although ceteris paribus a consider- 
able degree of intensity is favourable to the exhibition of the brushes. 
These observations account at once for the colour of the brushes seen with ordi- 
nary daylight. Inasmuch as no brushes are seen with the Jess refrangible colours, 
and the brushes seen with the more refrangible colours consist in the removal 
of a certain quantity of light, the tint of the brushes ought to be made up of red, 
yellow, and perhaps a little green, the yellow predominating, on account of its 
greater brightness in the solar spectrum. The mixture would give an impure yellow, 
which is the colour observed. ‘The blueness of the side patches may be merely the 
effect of contrast, or the cause may be more deeply seated. If the total illumination 
perceived be independent of the brushes, the light withdrawn from the brushes must 
be found at their sides, which would account, independently of contrast, both for 
the comparative brightness and for the blue tint of the side patches. 
The observations with homogeneous light account likewise for a circumstance with 
which the author had been struck, namely, that the brushes were not visible by 
candlelight, which is explained by the comparative poverty of candlelight in the 
more refrangible rays. The brushes ought to be rendered visible by absorbing a 
certain quantity of the less refrangible rays, and accordingly the author found that 
a blue glass, combined with a Nicol’ sprism, enabled him to see the brushes very 
distinctly when looking at the flame of a candle. .The specimen of blue glass which 
showed them best, which was of a tolerab!y deep colour, gave brushes which were 
decidedly red, and were only comparatively dark, so that the difference of tint be- 
tween the brushes and side patches was far more conspicuous than the difference 
of intensity. This is accounted for by the large quantity of extreme red rays which 
such a glass transmits. That the same glass gave red brushes with candlelight, 
and dark brushes with daylight, is accounted for by the circumstance, that the ratio 
which the intensity of the transmitted red rays bears to the intensity of the trans- 
mitted blue rays is far larger with candlelight than with daylight. 
An Attempt to explain the occasional distinct Vision of Rapidly Revolving 
Coloured Sectors. By Prof. στένει, LL.D. 
The author exhibited an instrument for whirling cards with coloured sectors on them, 
devised by Mr. Grattan of Belfast, to teach his children the effect of combining colours. 
He had shown this at the Natural History Society with an application for enabling 
painters to determine, experimentally, the mixture. of any number of colours, and 
