690 EAEL OF EOSSE ON THE CONSTEHCTION OF SPECULA OF 6-FEET APEETUEE, 
centre of the ball is in the centre of gravity of the triangle, not merely as respects its 
plane, but thickness also. These three triangles, which we call primary, carry at their 
angles, by ball-and-socket joints, nine secondary triangles, supported at their respective 
centres of gravity ; and they, in a similar way, carry twenty-seven tertiary triangles, each 
carrying three gun-metal balls of 1^ inch diameter, — in all, eighty-one balls, which sup- 
port twenty-seven equal portions of the speculum. Between the balls and the speculum 
twenty-seven thin brass plates are interposed, attached to the speculum by pitched cloth, 
not so much Avith a Adew of giving support between the balls, which Avould probably be 
quite unnecessary, but to make a smooth surface for the balls to roll upon without 
grinding the back of the speculum true. In each ball there is a small hole, and a thin 
brass wire is inserted in it, and secured with a wooden peg ; this Avire passes through a 
small hole in the lever, and is attached to a thin brass spring at the back, which yields 
as the ball rolls, and brings it back to its proper place AvheneA^er the ball is free from 
pressure. Without this contrivance, a very slight jerk, when the plane of the specu- 
lum is nearly vertical, would cause the balls to fall from their places. In practice, the 
motion of the balls is of course very slight. 
It is evident that so long as the speculum is horizontal, equal portions are carried 
equably, and it is almost as free from strain as if it was floating on mercury. As soon, 
however, as Ave incline the speculum to the horizon, the leA'er apparatus does not act so 
perfectly. It will have been observed that the levers are not in the same plane ; and 
this is disadvantageous in two ways : first, although the primary triangles balance in 
every position on the ball-and-socket joints, and therefore are indifferent as to position, 
the centres of the ball-and-socket joints carrying the secondary triangles are unavoidably 
above the plane of the centres of the primary supporting balls, and still more are the 
ball-and-socket joints carrying the tertiary triangles; consequently the secondary and 
tertiary triangles, by their weight, exert a force tending to make the planes of the pri- 
mary triangles rotate in a vertical plane, and so disturb the equilibrium. The tertiary 
triangles in a similar manner, but to a much less injurious extent, act upon the secon- 
dary triangles. The lever apparatus, deducting the primary triangles, AA^eighs about 
600 lbs. ; Avere it lighter it Avould not have the necessary solidity ; and the disturbing 
action of the weight is so considerable that, when not counterbalanced by subsidiary 
contrivances, the action of the speculum at low altitudes is much impaired by it. The 
contriA^ance we employ is a system of leA'ers, connected by Avires with the ball-and-socket 
joints which support the secondary triangles, and acting at right angles to the plane of 
the speculum. The primary triangles, thus relieved from all lateral strain, are in a 
condition to do their duty effectiA'ely ; and that seems to be sufficient in practice. Of 
course another set of levers might be attached to the ball-and-socket joints supporting 
the tertiary triangles ; and then the whole system AA'ould be perfectly counterpoised in 
eA'ery position ; that, however, seems to be scarcely necessary. It is evident that, Avere 
it not for the balls interposed betAveen the leA^ers and the speculum, any lateral motion 
of the speculum Avould introduce a disturbing force which Avould destroy the equi- 
