142 Lord Oxmantown on the construction of 



ficult double stars. A six inch metal was constructed about 

 a year ago upon the same principle, which performed well ; 

 and, upon the whole, I have not been able to discover that 

 these plated specula are subject to any defects to which those 

 upon the common construction are not equally hable. 



It is evident that such specula can be constructed of the 

 finest metal and of any size which may be desired, and that 

 with the greatest facility. A metal upon this plan two feet 

 diameter was commenced a few days ago, and is in so forward 

 a state that it will probably be completed in three weeks. A 

 second speculum of the same dimensions could be completed 

 in a shorter time, as no fresh tools would be necessary. 



This metal is for a tube twenty-six feet long and three feet 

 diameter. The tube is finished and the stand is nearly so. I 

 propose to make another metal for it at a future time of the 

 full aperture ; but whether single, or upon the plan described 

 in Number 17 of this Journal, I have not yet determined. 

 Further experiments are to be tried with the six inch metal, 

 the subject of the article before quoted. A few months after 

 that article was sent to the press, an eighteen inch metal of 

 twelve feet focus upon the same plan was commenced ; the dif- 

 ferent parts were cast. In the meantime some further expe- 

 riments were tried with the six inch metal ; the power of ad- 

 justment afforded much facility for comparing the spherical 

 aberration with the defects proceeding from other causes. By 

 retaining but a section of the aperture the spherical aberra- 

 tion was preserved, while the defects arising from inaccuracy 

 of surface were reduced with the diminished aperture. The 

 distinctness of the images increased as the surface lessened, 

 and the image resulting from the union of the two images was 

 as distinct as each had been when separately examined. Up- 

 on the whole, it appeared evident, that, although the speculum 

 was improved by the adjustment for spherical aberration, still 

 defects continued, arising from the imperfections of its surface, 

 much greater than the spherical aberration. The speculum 

 was repolished by hand with the utmost care more than fifteen 

 times, but without any considerable improvement. It was com- 

 pared when taken from the polisher with a common speculum 

 of the same dimensions, and they were found to be both alike. 



