In this new instrument, no tubes are necessary, as in the ordinary
opera-glass; their place being supplied by a slender elastic conical
spring of wire, into the upper extremity of which is inserted the
eye-glass; the object-glass being fixed to the other extremity, as
shown in the engraving. The two glasses must, of course, be kept
parallel to each other when in use; which is very easily effected.
[Illustration]
In using this opera-glass, rest the finger and thumb of one hand on
the rim of the object-glass, B, whilst, with the thumb and finger of
the other hand you hold the rim of the eye-glass, A. The spring tube
may then be drawn out or shut up to very minute distances. Thus, the
ordinary sliding tubes are superseded; nor is any external covering
necessary, as the hand in grasping the instrument serves the purpose.
If, however, a covering be preferred, a piece of silk may be sewn to
the spirals of the spring.
This kind of opera-glass may be made very cheaply; it may be shut into
a small space for the pocket, merely by pressing the object-glass and
eye-glass together.