TRANSACTIONS OF THE SECTIONS. ls 
In many of these lenses the lamine are separated in some parts and not in others, 
and the consequence of this is, that when we look at the convex surface of the para- 
boloid by reflected light, we observe a number of luminous and dark rings surround- 
ing the axis of the lens, the luminous rings being produced by the total reflexion of 
the light from one or more of the separated surfaces. The effect thus produced is 
very beautiful, as will be seen in the specimens on the table. 
The preceding observations were made, as I have already stated, nearly twenty- 
five years ago, and I have copied them as they stand in the Journal of my Experi- 
ments. I observe, however, some discrepancies between the figures and their de- 
scriptions, arising, I think, chiefly from not being able to distinguish the lenses of 
one species from those of another. These discrepancies I hope to be able to recon- 
cile before this communication is published. 
On the Use of Amethyst Plates in Experiments on the Polarization of Light. 
By Sir Davin Brewster, K.H., F.R.S. L. & F. 
In order to determine the exact position of the plane of primitive polarization, it 
was usual to observe when the intensity of the extraordinary image of the analysing 
prism was a minimum; but as it is difficult to obtain light perfectly homogeneous, 
the light of this image could not be completely extinguished. In his experiments on 
the rotatory phenomena of quartz, M. Biot employed a coloured glass, which trans- 
mitted only the extreme red rays of the spectrum; but this method, owing to‘the 
great loss of light in the polarized pencil, was attended with so many incouveniences, 
that fifteen or twenty trials were required before he could determine the zero of his 
instrument. In order to remedy this evil, M. Soleil interposed between the polar- 
izing apparatus and the analysing prism two plates of quartz of equal thickness, the 
one right-handed and the other left-handed. These plates were united so as to give 
the same tint when the plane of the principal section of the analysing prism coin- 
cided with the plane of primitive polarization. This ingenicus apparatus was sub- 
mitted to the Academy of Sciences on the 23rd of June, 1845, and has been used 
since that time by M. Senarmont and others in their experiments on polarization. 
In the year 1819 I communicated to the Royal Society of Edinburgh the very same 
method of placing the principal section of the analysing prism in the plane of pri- 
mit:ve polarization ; but in place of using two plates of right and left-handed quartz, 
I used a single plate of amethyst, in which the two kinds of quartz were combined, 
during the formation of the crystal. This piece of apparatus, which is obviously 
superior to that of M. Soleil, is thus described in the paper to which I have referred :— 
“The properties of amethyst, which have now been described, render a plate of this 
mineral a valuable addition to our apparatus for conducting experiments on the 
polarization of light. If we wish to place the principal section of the analysing 
prism exactly in the plane of primitive polarization, we have only to interpose a thin 
plate of amethyst like that shown in the figure, and if the tints of both sets of veins 
are exactly similar, the analysing prism will have the required position. If the one 
set of tints is bluer or whiter than the other, or if there is the slightest difference 
between them, the position of the prism must be altered till that difference is no 
longer perceptible. If we wish to place a plate of sulphate of lime or any other 
crystal, so as to have its principal section in the plane of primitive polarization, the 
interposition of the amethyst plate will give us the same assistance, by indicating 
that the circular (rotatory) tints are not affected by it; whereas if we wish to place 
the axis of the sulphate of lime at an angle of 45° to the primitive plane of polariza- 
tion, the amethyst will point out this position when the opposite circular tints suffer 
an equal change.” 
On Professor Petzvay's New Combination Lens. 
Sir Davin Brewster laid before the meeting a paper, translated by Mr. Paul 
Pretsch, entitled ‘ Prof. Petzval’s New Combination Lens, as an Object-glass for 
Telescopes,’ and exhibited a fine telescope for which the lens had been originally 
constructed. This telescope was constructed for the Imperial General Survey Office 
at Vienna, for the purpose of making maps, and the lens which it contained has been 
