76 
King AID, on a New Adjustable Diaphragm. 
by brass rings, E, fixed with small screws, a tube of vulcanized 
india-rubber, F, equal in length to A, but rather less in 
diameter than the interior of C. By turning the milled 
edge while A remains fixed, the rubber is made to extend 
inwards, and when half a rotation is accomplished, it com- 
pletely closes the aperture, which remains constantly central, 
and nearly circular, being, indeed, a polygon of a great 
Pig. 1. . Fig. 2. 
number of sides. The principle of the contrivance will, 
perhaps, be more readily understood by a reference to Fig. 2 ; 
for if two rings, A and B, be supposed similarly divided in 
any number of points, and those points connected by parallel 
lines forming a skeleton cylinder, it will be seen at once that 
if one of the rings be turned half way round, every line will 
pass through the centre, C, of such cylinder, the general 
figure resembling an hour-glass. The india-rubljer tube must 
evidently be considered as composed of an infinite number of 
such threads. 
In Fig. 2 is given a sketch of the appearance presented by 
the rubber when the opening is nearly closed, only six folds, 
however, being taken for the purpose of illustration. 
In conclusion, I would remark the necessity of making the 
tube C sufficiently long in proportion to its diameter, to allow 
of the rubber quite closing the aperture. The cement com- 
monly used by opticians in the construction of electrical 
a2:>paratus seems well suited for fixing the rubber to brass, 
and a solution of caoutchouc in mineral naphtha may be used 
to join the surfaces of rubber. 
