502 
EARL ROSSE’S OBSERVATIONS ON THE NEBULAE. 
speculum metal and wrought iron is about five to six three-tenths ; yet strange as it 
may appear, so delicate is the optical test, that strong pressure of the hand at the 
back of a speculum, four tons weight, and nearly six inches thick, produces flexure 
sufficient to distort the image of a star. It is obvious, therefore, that a slight in- 
equality in the action of the lever apparatus supporting a 6-feet speculum would pro- 
duce an amount of flexure sufficient to destroy definition. It has not been found pos- 
sible so to secure the 6-feet specula as to prevent a slight change of place in a plane 
parallel to the plane of the levers, and as the levers are not all in one plane as in the 
case of the 3-feet specula, and a considerable amount of friction exists between the 
speculum and its lever supports, when the speculum changes place, however slightly, 
there will be a force tending to disturb the equal actions of the levers. It has been 
found that when the speculum changes its place one-thirtieth of an inch, still adhe- 
ring to its levers, unmistakeable distortion will be produced. We have occasionally 
observed, even during a night’s vmrk, the sudden appearance, and the as sudden dis- 
appearance of the rudiments of focal lines, the undoubted evidences of flexure ; but 
we have not found that flexure, even to the extent of materially disfiguring the image 
of a large star, interferes much with the action of the speculum on the faint details of 
nebulse, although it greatly lessens its power in bringing out minute points of light, 
and in showing resolvability where under favourable circumstances resolution had 
been previously effected. 
In the spring of 1848 the heavier of the two specula for nearly three months per- 
formed admirably, very rarely exhibiting the slightest indication of flexure. It then 
remained inactive for some time before and after the solstice, and when we again 
commenced observing it was found to be in a state of strain ; the friction between the 
lever apparatus and the speculum had no doubt in the meantime increased consider- 
ably, and the levers being therefore unable to adjust themselves to some slight but 
permanent change in the place of the speculum, they no longer supported it equably. 
It was cautiously raised a little by screws for the purpose of re-adjusting the levers, 
and to our surprise the unequal strain of the screws was found to have produced per- 
manent flexure, so that the speculum did not again perform well till after it had been 
reground. From the experiments of Mr. Eaton Hodgkinson and others, we should 
have been prepared for a change of figure in a mass of cast iron, but with a material 
so brittle and so elastic as speculum metal, the result was quite unexpected. Re- 
cently, in supporting the lighter of the two specula, twenty-seven triangles have been 
substituted for the twenty-seven platforms, each triangle carrying at its angles three 
brass balls, so that the speculum rolls freely on eighty- one balls, which support it 
pretty nearly equably. This appears to be a great improvement, but I will not dwell 
further on the subject. To describe the experiments which have been made with a 
view of discovering the best means of supporting very large specula, a question of 
great theoretical and practical difficulty, would occupy too much space, and would 
require elaborate engravings ; it would besides be foreign to the object of this paper. 
