British Association for the Advancement of Science. 2% 
plate of glass before his eye, in such a manner as to intercept and 
retard one half of the pencil, which was entering his eye, by 
placing it before one half of the pupil. He was then surprised to 
find, that when the edge of the retarding glass plate was turned 
towards the red end of the spectrum, intensely black lines made 
their appearance, as might be expected, at such regular intervals, 
as to represent the most exact micrometrical arrangement of wires; 
but upon turning the plate of glass half round, (still keeping its 
plane perpendicular to the axis of the eye,) so as to present the 
edge, past which the rays entered the eye, to the violet end of 
the spectrum, every one of those dark bands entirely disappeared. 
In the intermediate positions of that edge they appeared more or 
less distinct, according as the edge was more presented to the red, 
or to the violet end of the spectrum. <A glass plate, one-thirtieth 
of an inch thick, gave these lines ; but the thinner the glass, the 
more intense was the blackness, and the more distinct the lines. 
They were formed in any part of the spectrum; but they were 
best seen when the rays were intercepted which lay between the 
two fixed lines A and D, of Frauenhofer. An examination of 
these lines aflorded the very best means of determining the dis- 
persive powers of substances, for their distance from one another 
inereases or diminishes, exactly as the entire length of the spec- 
trum is increased or diminished ; and the number of them in the 
same part of two spectra is always the same. 
Comparative Composition of Cast Iron prepared with the hot — 
and the cold blast.—Dr. Thomson observed, that the specimens 
of cast iron examined, were all from iron smelted from the iron- 
stone in the Glasgow coal-field. This iron-stone is a carbonate 
of iron, more or less pure. 'The richest is known by the name of 
Mushet’s black band, which occurs in the neighborhood of Air- 
ae ; ; its specific gravity is 3.0553, and it is composed of 
Carbonate of iron, - - §85.44 
- of lime, - ~ . 5.94 
“ of magnesia, a 3.71 
Si iaee sieees tans 08 
Peroxide of iron, - ~ - rea 
