TRANSACTIONS OF THE SECTIONS. TS 
i 
spectrum formed in the focus of an achromatic telescope, after the 
anner of Fraunhofer, he placed a thin 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 re- 
lar intervals, as to represent the most exact micrometrical arrange- 
nent 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 pre- 
sent 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 in- 
tense 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 Fraunhofer. An examination of these lines afforded the very 
_ best means of determining the dispersive powers of substances; for 
_ their distance from one another increases or diminishes, exactly as the 
4 entire length of the spectrum is increased or diminished ; and the 
number of them in the same part of two spectra of different lengths i is 
_ always the same. 
_ Notice of a new Structure in the Diamond. By Sir D. Brewster. 
__ Sir David said, that having communicated to the Geological Society 
an account of certain peculiarities in the structure of the diamond, 
; Rrchich confirm the theory of its vegetable origin, he was desirous of 
_ submitting to the consideration of this section a new structure, which 
_ he had recently detected in that gem, and which indirectly supported 
_ the same views. In consequence of the diamond having been used as 
the fittest substance for forming single microscopes of high power and 
_ small spherical aberration, the attention of opticians has been drawn 
_ to the imperfections of its structure. Mr. Pritchard, who first suc- 
_ ceeded in executing lenses of diamond, put into the hands of Sir David 
_ for examination, a plano-convex lens, about the 30th of an inch in 
_ diameter, which he had found unfit for the purposes of a microscope, 
in consequence of its giving double or triple images of minute objects. 
_ As Sir David had previously shown that almost all diamonds possessed 
- an imperfect doubly refracting structure, as if they had been aggregated 
_ by irregular forces, or compressed or kneaded together like a piece of 
soft gum or an indurated jelly, he had no doubt that the multiple images 
_ were owing to this structure, as there appeared, on an ordinary ex- 
- amination of the lens, to be no other cause to which it could be 
