6 REPORT—1849, 
On a new form of Lenses, and their Application to the Construction of two 
Telescopes or Microscopes of exactly equal Optical Power. By Sir Davin 
Brewster, A.W, D.C.L., PRS. § V.PRS. Edin. 
This method is to divide, in the same way as before described, one lens into two 
semi-lenses or quadrants, or sextants or octants, and using each semi-lens or quadrant 
for forming the image. Sir David also showed, that by a proper combination and 
adjustment of two such semi-lenses or quadrants in a frame or tube, by placing 
their diameters at a proper angle, each may be made to correct the imperfect image 
formed by the other. 

Notice of Experiments on Cireular Crystals. 
By Sir Davin Brewster, KA, D.C.L., F.R.S., §& ViPRS. Edin. 
Mr. Fox Talbot first studied the phenomena of this class of crystals as exhibited 
in those produced by a mixture of borax and phosphoric acid, and Sir David Brewster 
exhibited to the Section drawings of this phenomenon which kad been presented to 
him by Mr. Fox Talbot. In the course of his own inquiries he discovered a large 
number of bodies which yielded circular crystals, which he divided into two classes, 
positive and negative, including oil of mace (the phenomena of which he had pre- 
viously described in the Phil. Trans. for 1814), animal fat, wax, &c., in which it is 
very difficult to distinguish circular from quaguaversus polarization. 
Additional Observations on Berkeley's Theory of Vision. 
By Sir Davin Brewster, K.H., D.C.L., F.RS., §& V.PRS. Edin. 
Tn this paper, the author, by various arguments, partly metaphysical, partly optical, 
and by examining the accounts of persons couched for cataract at an advanced period 
of life, which were relied on by the supporters of Berkeley’s views (illustrating his 
views by several diagrams), considered he had overthrown every position essential to 
the maintenance of that theory, and especially the fundamental proposition from which 
that philosopher started. 

An Account of a new Stereoscope. 
By Sir Davip Brewster, A.H., D.CL., P.RS., & ViPRS. Edin. 
The ingenious stereoscope, invented by Professor Wheatstone for representing 
solid figures by the union of dissimilar plane pictures, is described in his very in- 
teresting paper ‘‘On some remarkable and hitherto unobserved Phenomena of 
Binocular Vision ;”” and in a paper published in a recent volume of the Edinburgh 
Transactions, Sir David Brewster has investigated the cause of the perception of 
objects in relief, by the coalescence of dissimilar pictures. Having had occasion to 
make numerous experiments on this subject, he was led to construct the stereoscope 
in several new forms, which, while they possess new and important properties, have 
the additional advantages of cheapness and portability. The first and the most 
generally useful of these forms is the Lenticular Stereoscope. This instrument con- 
sists of two semilenses placed at such a distance that each eye views the picture or 
drawing opposite to it through the margin of the semilens, or through parts of it — 
equidistant from the margin. The distance of the portions of the lens through which 
we look, must be equal to the distance of the centres of the pupils, which is, at an 
average, 22 inches. The semilenses should be placed in a frame, so that their di- 
stance may be adjusted to different eyes. When we thus view two dissimilar draw- 
ings of a solid object, as it is seen by each eye separately, we are actually looking 
through two prisms, which produce a second or refracted image of each drawing, and 
when these secend images unite, ur coalesce, we see the solid object which they re- 
present. But in order that the two images may coalesce without any effort or strain 
on the part of the eyes, it is necessary that the distance of similar parts of the two 
drawings be equal to twice the refraction produced by each lens. For this purpose, 
measure the distance at which the semilenses give the most distinct view of the 
drawings, and having ascertained, by using one eye, the amount of the refraction 
produced at that distance, or the quantity by which the image of one of the draw- 
ings is displaced, place the drawings at a distance equal to twice that quantity, 

