to Mm by the Kev. T. E. Robinson, D.D., Armagh Observatory. 

 Before proceeding to describe the model, Professor Wilson gave a 

 brief account of the principle of the reflecting telescope, as contrasted 

 with the refracting telescope, with a description of the various con- 

 structions used since the time of Newton. After stating that a specu- 

 lum afforded only as much light as a lens of about three-fourths its 

 aperture, he went on to say, that the impossibility of procuring glass 

 of a sufficiently uniform texture rendered it necessary to make use of 

 reflecting telescopes when any very great optical power was required. 

 The construction which it was proposed to use in this telescope was 

 that recommended by Cassegrain, with this important improvement, 

 that the small convex mirror was formed of an achromatic combina- 

 tion of lenses coated with a deposit of pure silver. It was found 

 that this reflected 86 per cent, of the incident bight, whilst speculum 

 metal reflected only 62 per cent. The speculum, which was to be of 

 a clear aperture of four feet, would weigh about one ton, and the 

 whole moving part of the instrument would weigh something more 

 than eight tons. The mode of mounting adopted, so as to secure at 

 the same time ease of motion and perfect steadiness, formed the 

 most beautiful part of the contrivance. The modes of mounting 

 hitherto used might be divided into two classes, which might be 

 termed the English and the German modes of mounting. In the 

 English mounting the polar axis was longer than the telescope, and sup- 

 ported at both ends. This had the disadvantage that the polar axis, 

 from its great length, was deficient in rigidity, and also that the view of 

 the sub-polar portion of the heavens was partially interrupted by the 

 support of the upper end of the axis. In the German mode of 

 mounting the polar axis was supported only below the centre of 

 gravity of the telescope, and consequently any flexure would have a 

 greater tendency to produce an angular derangement of the instru- 

 ment than a much greater flexure in the English mode. The mode 

 proposed for this instrument combined the advantages of both. The 

 polar axis was very short, and was supported at both ends, while the 

 telescope commanded an uninterrupted view of every portion of the 

 heavens. The weights of the various parts, also, were so counter- 

 poised that there would be very little wear of the various bearing 

 Y's from friction. Though the moving parts weighed more than 

 eight tons, it was calculated that a force . of twenty pounds, acting at 

 a radius of five feet, would be sufficient to move the instrument in 

 right ascension. The telescope was to be provided with a clock-work 

 movement, so that when it was once directed to a star, the star would 

 remain in the field of view without further exertion on the part of the 

 observer. It is impossible to render the construction intelligible 

 without diagrams. Professor Wilson, however, stated that he should 

 be happy to explain it to any person taking an interest in it, who 

 would pay the model a visit at the University. Professor Wilson 

 read the tender of Mr. Grubb, of Dublin, offering to construct this 



