32 On the Halo seen around all Bodies. 
45. The lenticular character of the halo is indisputable, and 
every experiment will the more fully establish the fact. Jt is a - 
true reflecting and magnifying medium, and it is entirely owing 
to this circumstance that a conver lens of glass possesses its mag- 
nifying power. The mere substance or material of glass is only 
necessary or accessory to the production of lenticular phenomena 
in consequence of its capability of curvature and extension. 
46. Therefore, beyond a certain point a halo cannot maintain 
its continuity, unless it have a solid or fluid medium on which it 
can expand and keep its particles in contact. Nor can its ulti- 
mate magnifying power be developed unless the material in which 
it rests is conver, for being convex itself it requires a continued 
extension of convex surface if greater magnifying power is re- 
quired. An aperture which is of twice the diameter of the halo 
will have an open space in the center free from it. This center, 
therefore, is no lens, but if we put a convex glass in the aperture, 
the halo then has a conducting medium, and can spread itself, or 
rather connect itself with the halo on the glass, and thus exhibit 
all the powers of a lens. 
47. It is the continuity of the halo throughout all the space of 
a small circular aperture which gives the aperture, or the convex — 
glass within it, the character of alens. In consequence of this 
continuity, lenses can be built up of many pieces, and of course — 
there is no limit to their diameter. Converity being the sole re — 
quisite for a magnifying power, it is immaterial whether the lens | 
be of solid glass, or whether the two convex surfaces be of the © 
thinnest glass, cemented at the circumference and the hollow — 
space filled with a fluid. If the lens be built up of many pieces, — 
the blocks should all run parallel with the axis of the glass and — 
the central block should be of one piece throughout its axis, 80 — 
that there may be no interruption of the rays of light through it. 
It is of no consequence how narrow the diameter of the block — 
may be, for the rays which are to give impressions of external 
objects converge to a minute point on the apex of this block and — 
pass in a straight line to the apex of the axis of our own eye: the 
present theory among philosophers is that the rays cross each 
other in the centre of the lens, but this is an error which will | 
pons ae] 
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