22 REPORT— 1866. 



The Table contains a list of such pairs of stars, drawn up by Professor Grant of 

 Glasgow Observatory ; and with each pair there is given a correction-factor, which, 

 being multiplied by the secaut of the latitude of the place of observation, gives the 

 correction of the approximate meridian found by the process ; that is to say, the 

 true azimuth of a plane passing thi-ough the observer and the two stars at the 

 instant when that plane is vertical. The following are the formidse for the correc- 

 tion-factor C and its use : — 



Let PA and PB be the polar distances of the two stars, D the difference of their 

 right ascensions in seconds of angle j then 



D 



C = 



cotan PA — cotan PB ' 



and if Z be the correction in azimuth in seconds of angle, and L the latitude of the 

 place, 



Z=C.secL. 



Instruments. 



On some Recent Improvemenis in Astronomical Telescopes with Silvered Glass 

 Specula. By John Bro"\vning, F.R.A.S. 



During the last year I have devoted a large portion of time to an attempt to 

 improve the construction of telescopes moimted with silvered glass specula. The 

 methods usually adopted for mounting the speculum are, 1st, suspending it ; 

 2ndly, supporting it upon an air-cushion, or a bed of felt ; 3rdly, allowing it to rest 

 on a number of balanced triangles. 



All of these plans are open to the objection that the specula, when thus moimted, 

 are very liable to get out of adjustment. The plan I have adopted consists in 

 bringing the bottom of the speculum to a veiy accurate plane surface, and then 



E lacing it in an iron cell, the inner surface of the cell on which the speculum rests 

 eing also made an accurate plane. Moimted in this manner, the speculum can 

 be removed fi'om and replaced in the telescope without fear of deranging its 

 adjustment. 



The adjusting screws must be an-anged so that they do not throw any strain on 

 the iron cell, which would be liable to produce flexure in the speculum. Many 

 specula have been mounted by this method, up to 10 inches diameter, with success. 



It is known that the arm which carries the diagonal mirror in reflecting tele- 

 scopes produces coarse rays on bright stars. Instead of the usual arm, I mount 

 the diagonal mirror by means of three strips of chronometer spring strained tightly 

 edgewise towards the speculum. 



When observing stars with reflectors, the diffraction-rings sm-rounding them are 

 greatly reduced by employing with all Huyghenian eyepieces a good Barlow lens. 



Achromatic eyepieces (not the Kellner construction) give superior results to 

 Huyghenian. 



lube-currents can be almost entirely avoided by making the body or tube of 

 iron, which quickly equalizes the temperature of the air inside and outside 

 the tube. 



Should an observatory be built for a reflector, it should be made of sheet-iron, 

 as when constructed of this material the temperature inside and outside the 

 building wiU be always nearly alike, and annoying air-cuiTents, which would be 

 generated in a building constructed of a non-conducting material, will not occur. 



Peflecting telescopes require steadier stands than refractors, and the importance 

 of seciu'ing this steadiness can scarcely be overrated. 



With reflectors mounted as described, if the precautions which have been enu- 

 merated are adopted, the performance wiU almost rival that of good refractors, 

 and in di\'iding power excel them. 



The Moon Committee of the British Association has recommended that ob- 

 servations upon the moon should be made with a power of 1000 diameters. 

 Such a power can only be applied in telescopes of large aperture, which are very 

 costly. 



