306 Royal Society. 



common wire micrometer with thick lines has been found to succeed 

 the best ; for the faint details of the nebulae are extinguished by any 

 micrometrical contrivance which either diminishes the light of the 

 telescope or renders the field less dark ; and thick lines have been 

 found to be visible without illumination in the darkest night. 



The telescope has two specula, one about three and a half, and 

 the other rather more than four tons weight. Each is provided with 

 a system of levers to afford it an equable support. Upon this 

 system it was placed before it was ground, and has rested upon it 

 ever since. The systems of levers with the mode of applying them 

 in the support of the speculum are described in the paper, and also 

 the precautions taken to guard against strain and consequent flexure 

 of the metal. Notwithstanding these precautions, undoubted evi- 

 dences of flexure in the speculum have occasionally shown them- 

 selves. It has not, however, been 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 nebulae, 

 although it greatly lessens its power in bringing out minute points 

 of light, and in showing resolvability where, under favourable cir- 

 cumstances, resolution had been previously effected. 



It is stated that, in the spring of 1848, the heavier of the two 

 specula, for nearly three months, performed admirably, very rarely 

 exhibiting the slightest indication of flexure. It then remained in- 

 active for some time before and after the solstice, and when obser- 

 vations with it were again commenced, it was found to be in a state 

 of strain. On cautiously raising it a little by screws, for the pur- 

 pose of readjusting the levers, it was found that the unequal strain 

 of the screws had produced permanent flexure, so that the speculum 

 did not again perform well until after it had been reground. Re- 

 cently an alteration has been made in the mode of supporting the 

 lighter of the two specula, which now rolls freely on eighty-one 

 brass balls that support it nearly equably. After referring to other 

 causes of unequal action, among which the varying state of the 

 atmosphere is one of the most serious, the author remarks that the 

 Society will not be surprised should it be in his power at a future 

 time to communicate some additional particulars, even as to the 

 nebulae which have been most frequently observed. 



The very beautiful sketches which illustrate the paper, are, it is 

 remarked, on a very small scale, but are sufficient to convey a pretty 

 accurate idea of the peculiarities of structure which have gradually 

 become known. In many of the nebulae they are very remarkable, 

 and seem even to indicate the presence of dynamical laws we may 

 perhaps fancy to be almost within our grasp. 



On examining these sketches it will at once be remarked, as stated 

 by the author, that the spiral arrangement so strongly developed in 

 H. 1622, 51 Mesier, is traceable more or- less distinctly in several of 

 the sketches. More frequently indeed there is a nearer approach 

 to a kind of irregular interrupted annular disposition of the luminous 

 material, than to the regularity so striking in 51 Mesier; but it can 

 scarcely be doubted that these nebulae are systems of a very similar 



