Dr Brewster on the Construction of Polyzonal Lenses , §c. 161 
2. From the difficulty of procuring such a large mass of 
glass'free from imperfections ; and* 
8. From the great thickness which is necessary at the centre 
of a large lens, in consequence of which a very con- 
siderable quantity of light and heat is absorbed, and 
scattered, by the numerous veins and imperfections 
which occur in such a thickness of transparent matter. 
The celebrated Buffon proposed to remedy the last of these 
evils by what he calls Lentilles en echelons , or lenses with steps, 
as shewn in Plate IV. Fig. 1., where the lens ACBEFGBjg/bdc, 
consisting of one piece of glass, will produce the same effect as 
the lens ACmG^gnCj and yet does not contain one-half the 
quantity of glass. The difficulty, however, of grinding the sur- 
faces CD, DEF, EG, &c., independently of the difficulties 
above enumerated, rendered it impossible to execute such lenses 
on a large scale, so that the idea of M. Buffon must be con- 
sidered as one of those ingenious ‘ but visionary speculations 
which never terminate in practical utility 
In the year 1811, the idea occurred to me of constructing large 
lenses of many zones or rings , and of composing each zone of 
separate segments, so that a lens of any magnitude might be built 9 
as it were, of separate pieces ; and in 1812, I published, in the 
article already quoted, the following method of construction •)*.] 
<fi In order to remove these evils, and at the same time to di- 
minish the expence, and simplify the construction of dioptric burn- 
ing instruments, the following construction has been proposed by 
Dr Brewster. If it be required, for example, to construct a burn- 
ing lens 4 feet in diameter, it should be composed of different 
pieces, as represented in Plate IV. Fig. 2., where ABCD is 
a lens of flint-glass, 18 inches in diameter. This lens is sur- 
rounded by several segments, AGID, AGEB, BELC, CLID, 
ground in the same tool with ABCD, but so formed with re- 
spect to their thickness at AB and GE, &c. that they may ex- 
actly resemble the corresponding portions of a solid lens. These 
* It is quite certain that Buffon constructed a lens of one piece of glass from 
12 to 15 inches in diameter, according to this principle. 
This mode of constructing lenses is also referred to in my Treatise on Nev# 
Phil. Instruments , Edin. 1813, p. 399„ Note. 
VOL. VI1T. NO. 15. JANUARY 1828. L 
