British Association for the Advancement of Science. 21 



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 afforded the very best means of determining the dis- 

 persive powers of substances, for their distance from one another 

 increases 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 Mast. — 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 

 Mushefs black band, which occurs in the neighborhood of Air- 

 drie ; its specific gravity is 3.0553, and it is composed of 



Carbonate of iron, - - - 85.44 



" of lime, - - - 5.94 



" of magnesia, - - 3.71 



Silica, 1.40 



Alumina, - - - - - 0.63 



Peroxide of iron, - - - 0.23 



Coal, - - - - - 3.03 



100.38 



