for Light-Houses and for Burning Glasses . 3 67 
Note), I have long ago suggested the application of these views 
even to lenses of flint-glass, for the most delicate purposes in 
optics ; and there is some reason for expecting that lenses with 
steps, either in a solid state, or composed of zones and segments, 
and solid lenses built with separate pieces of glass, of the same 
refractive power, or with zones of variable density, will speedily 
be introduced into actual use * 
The great superiority of lenses over mirrors, for the purposes 
either of fixed or moveable lights in light-houses, is so manifest, 
that we expect to see them replacing the parabolic reflectors in 
all our light-houses ; but when we consider the scientific results 
which are likely to be obtained from improved burning instru- 
ments, too much labour cannot be bestowed in bringing them 
to perfection. 
“ The formation of instruments for burning at short distan- 
ces,’ 1 as I have remarked in the work already quoted ( Edin- 
burgh Encyclopedia , vol. v. p. 139.)? 66 is a subject of the first im- 
portance to science, and we have no doubt that they will, some 
time or other, be employed as the most powerful agents in che- 
mistry and the arts. Though catoptric burning instruments of 
great power have been constructed, yet their effects have never 
been so great as those of lenses, and they are besides liable to 
numerous disadvantages. The burning point must evidently 
lie between the centre of the mirror and the sun, and, therefore, 
the operator cannot so easily perform his experiments, as when 
the focus lies on the other side of the instrument. All his ope- 
rations, indeed, have a tendency to obstruct the light before it 
falls upon the reflecting surface. This evil will not admit of a 
remedy, and consequently we must have recourse to instruments 
of a dioptric or catadioptric nature, which are alone capable of 
uniting great power with great facility of management. 11 
In constructing a burning apparatus with a large lens, it has 
been found advantageous to use a second lens to receive and 
concentrate the converging rays from the principal lens. This, 
we believe, was first proposed by M. Tschirnhausen, and was 
• The same principle is applicable to the construction of large prisms for opti- 
cal experiments, as shewn in Fig. 7, where the same effect may be produced by the 
six small prisms AD, as by the whole prism ABC. These six prisms may be all 
ground in one prismatic rod, which may be cut into small prisms when polished. 
