182 REPORT—1850. 
of points, has materially diminished the evil. Still, when high powers are used, and 
in favourable states of the air, there is some evidence that it is not altogether removed. 
And the principal abject of this communication is to describe a method which I intend 
as soon as practicable to carry into effect ; and which will, I hope, remove all sensible 
distortion arising from this cause. 
I would propose to cast on the back of the speculum a series of bars, in number 
about seven, whose section shall be similar to the tooth of a saw, and which shall 
serve as points of attachment for a number of levers, having their fulcra in the plate 
which supports the speculum at its back. The disposition and mode of action of 
these levers will be immediately understood by inspection of the annexed diagrams. 
In fig. 2, a represents the speculum in section with its bars ὃ, ὃ, &c., d the back-plate 
affording support to the several levers c,c, &c. The ends of the levers 6 are mounted 
with friction-wheels to prevent any tension from contact. The shorter arm of the 
lever should be as short as possible, in order to diminish to a minimum the weight 
and length of the other arm. The number of these levers may be about thirty-five, 
equally distributed over its back; and, in a two-foot speculum of the estimated 
weight of 400lbs., may each be loaded so as to exert an upward pressure of about 11105. 
each, in order that the speculum may not be altogether on a balance, but sufficiently 
in contact with the supporting hoop to prevent dancing, and yet supported in so many 
points as to prevent all distortion of the lower half by the weight of the upper, or of 
the upper half for want of adequate support. It is evident that these levers have no 
action whatever when the telescope is directed to the zenith, in which position they 
are not required, and are only called into action in proportion as the telescope is de- 
pressed from the zenith, exerting their full force when the telescope is horizontal. 
Fig. 3 represents a view of the back of the speculum with its several bars, and the 
points where the friction-wheels of the levers come in contact with them. 
Fig. 2. Fig. 3. 
It may be objected that the casting of a speculum with these bars will be more 
difficult than without them, which I admit, though perhaps not in a very great de- 
gree. The bars however are not essential, for the requisite points of attachment of 
the levers may be obtained by cementing to the back of the speculum, with good 
fresh plaster of Paris, the requisite number of small pieces of speculum metal or alloy ; 
and I have ascertained by experiment that the adhesive power of this material is quite 
sufficient for the purpose. It will be necessary also practically to deviate a little from 
the symmetry of fig. 3 in the arrangement of the supporting points, the system of 
levers for the vertical support in some cases interfering; but this will not be to an 
injurious extent. 
