1864. | Physics. 499 
greater difference between these two temperatures in summer than in 
winter, although in the latter case the rays have to travel through 
about twice as much atmosphere. The explanation of this is, that 
there is more aqueous vapour in the air in summer than in winter, 
thus fully bearing out the observations of Professor Tyndall as to the 
power of the vapour of water to intercept the sun’s rays. 
A discussion has been going on between MM. Van Monkhoven 
and Bertsch on the possibility of constructing a system of lenses 
which will augment the intensity of the solar rays without changing 
their parallelism. M. Bertsch has proposed an arrangement intended 
to effect this object, consisting of a convex lens, on which the solar 
rays fall, forming an image of the sun in its focus. Between the lens 
and focus, however, a small concave lens is interposed, in such a 
position that the convergent rays, after passing through the second 
lens, emerge parallel and concentrated. A similar effect may be 
obtained by having a small convex lens of short focus placed beyond 
the focus of the larger lens. M. Van Monkhoven argues that 
although an apparatus of this kind would be applicable to light issu- 
ing from a luminous point at an infinite distance off, the instrument 
is inapplicable to sun-light, because this body has a sensible diameter ; 
and since the image formed at the focus of the larger lens would be 
necessarily of greater diameter than that formed by the other at its 
focus, no condensation can possibly take place. To this argument 
M. Bertsch replies, that he has made the instrument, and it does an- 
swer; and that as the angles of the pencils of rays proceeding from the 
sun never exceed half a degree, they are so small that they may be 
neglected. 
We have already * called attention to some researches by M. G. 
Quincke, on the optical properties of the metals, in which he showed 
that their refractive indices were less than unity ; in a second memoir 
by the same author on this subject, the theory is followed out mathe- 
matically as well as experimentally, and the further discovery is 
announced that the refractive index of the metals is dependent upon 
the angle of incidence and increases with an increasing angle. 
When a luminous body is viewed through some kinds of trans- 
parent minerals, such as certain varieties of mica, rays of light are 
seen to diverge from the luminous centre, at equal distances apart. 
This appearance has received the name of asterism. A method of 
producing asterism artificially, in a manner as clear and perfect as is 
met with in some of the naturally-occurring minerals, has lately been 
published by M. C. A. Griiel,f of Berlin. A clear piece of plate-glass 
is cut in the form of an equilateral triangle, with sides measuring 14 
to 2 inches. The surface of this triangle is then rubbed backwards 
and forwards a few times on a sheet of fine emery-paper ; each of the 
three sides being successively guided against a metal rule, which at 
the same time being pressed on the emery-paper, keeps it in position. 
* © Quarterly Journal of Science, vol. i. p. 542. 
+ ‘ Phil. Mag,’ series iv. vol. xxvii. p. 400. 
