964^ 



much with the action of the speculum on the faint details of nebulae, 

 although it greatlj- lessens its power in bringing out minute points 

 of light, and in showing resolvability w^here, under favourable cir- 

 cumstances, resolution had been previously effected. 



It is stated that, in the spring of 184'8, 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 sjDecuIa, 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 

 nebulas 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 nebulse 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 Messier, 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 Messier; but it can 

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

 nature, seen more or less perfectly, and vario\isly placed with refer- 

 ence to the line of sight. The author adverts to the description of 

 this nebula by Messier, Sir William Herschel and Sir John Herschel, 

 and remarks, that taking the figure given by Sir John, and placing 

 it as it would be seen with a New'tonian telescope, we shall at once 

 recognise the bright convolutions of the spiral which were seen by 

 him as a divided ring : thus with each increase of optical powder the 

 structure has become more complicated, and more unlike anything 

 which we could picture to ourselves as the result of any form of 

 dynamical law of which we find a counterpart in our system. After 

 pointing out the importance of measurements and the difficulty of 

 taking them satisfactorily, the author states, that of a few of the 

 stars with which the nebula is pretty well studded, measurements 

 with reference to the principal nucleus were taken by his assistant 

 Mr. Stoney in the spring of 1849, and that these have been repeated 

 this year during the months of April and May, and also some mea- 



