Proceedings of Learned Societies. 317 



for a 5-foot telescope, it was 331- inches between the hearings, 

 the central tnbe was 9 inches in diameter and 0*15 inch thick, 

 hut strengthened by internal webs, and weighed only 56 pounds. 

 The modifications in the adjustments related to three points : 1. 

 The adjustments for azimuth and horizontality. 2. The application 

 of the level. 3. The mode of collimation. 1. As to the raising 

 and lowering the axis, and moving it in azimuth. These opera- 

 tions are generally effected by sliding parts, which are objection- 

 able as they must be clamped, and this especially in the vertical 

 slide produces great strain. It is proposed that a massive three-rayed 

 piece of cast iron shall rest on each pier not fixed to it, but stand- 

 ing on three feet, with a central bolt to prevent accidental displace- 

 ment. This will admit of expansion in all directions. The bearings 

 in which the axis revolves rest on other three-armed pieces made of 

 gun metal, and standing on the iron ones. The gun metal piece 

 on one pier is acted on by screws having antagonistic motions, 

 which shift the bearings in azimuth as required. The other gun 

 metal three-armed piece has foot screws like a theodolite, for levelling 

 the axis. It has a circular level which is first adjusted, and the 

 serews are then moved as necessary, one of them having a lever 

 apparatus, by which great delicacy of motion is obtained. iNo 

 strain can exist under these arrangements, gravity being the only 

 force called into action. An improvement has also been made in 

 the bearings, the under surfaces of which constitute parts of 

 spheres ; and these work in cups, so that no amount of tilting will 

 interfere with the points resting securely in their bearings. 2. As 

 to the mode of applying the level to the axis. This is usually 

 effected by a level striding from one point to the other, but such 

 levelling cannot be performed with the telescope directed to the 

 zenith where error of level is most injurious ; and Mr. Cooke there- 

 fore proposes to have several levels attached to the telescope 

 instead. There will be two near the central tube, and one at each 

 end, all parallel to the axis. It is usually assumed that if the 

 points are horizontal the telescope will describe a vertical curve ; 

 but this is not a certain inference ; and it will be better to watch 

 the centre, where any unsymmetrical flexure of the axis will be 

 apparent, or any lateral deviation of the tube be made evident by the 

 end levels. 3. As to testing the collimation error. It is intended in 

 this instrument, which is a portable one made for the great Indian 

 survey, to employ Benzenberg's method of observing the reflection 

 of a wire cross in the eye- piece from a trough of mercury ; and in 

 order to avoid the vibrations of that fluid a plate of glass is to float 

 on the metal, the under surface of which will form a brilliant 

 mirror. Such plate ought to have two plane surfaces, parallel to 

 each other, and the glass should be of uniform density. The first 

 condition of a plane surface can be ensured by careful grinding ; 

 and should the other two not be obtained the reversal of the trough 

 in azimuth, and observations in that position, would neutralise the 

 error. 



In an interesting discussion which followed the reading of the 

 paper, Colonel Strange said the antagonistic screws of the azimuth 



