TRANSACTIONS OF THE SECTIONS. 53 



Several years before, Sir John Herscliel described a very different collimator for 

 adjusting rotiecting telescopes, wliicli consists of a Kater's collimator fastened 

 inside the tube of tlie telescope, parallel to its axis. In using this collimator tlio 

 eyepiece is not removed. The collimator emits a parallel beam of light, which, 

 falling on the speculum, enables the cross-wires to be seen as a distant object, 

 simultaneously with the heavenly body under re-view. 



Moreover the adjustments which the two instruments are capable of effecting 

 are also different. For while Sir John Herschel's collimator enables the observer 

 to bring the image of that point of the object towards which the axis of the tube of 

 his telescope is pointed, into the middle of his field of view, which is the adjustment 

 of most importance when the telescope is to be used as a surveyim/ instrument, Mr. 

 Stoney's collimator enables the observer, if his mirrors are out of adjustment, to 

 move the small mirror so as in the greatest possible degree to compensate for a faidty 

 position of the great speculum, which is the adjustment of most unportance, when 

 the telescope is used as an optical instrument. 



Thus the collimators themselves, and the adjustments they effect, are entirely dif- 

 ferent ; yet Sir John Herschel, after describing his collimator in the later editions 

 of the 'Outlines of Astronomy,' writes in the following words of Mr. Stoney's 

 communication : — "It is to be presumed that Mr. Stoney, in bringing before the 

 British Association in 1856 this application of the collimating principle as a novelty, 

 has been unaware of this its prior use, since he has not alluded to it. The direct 

 reference of objects to the collimating cross described in the text would seem to 

 have been overlooked by him." — Outlines of Astronomj', 8th edition, page 128. 



It is true that Mr. Stoney was not aware in 1856 that Sir John Herschel had 

 suggested and used a different collimator for effecting other adjustments ; but it is 

 equally true that manj^ readers have been misled by the foregoing passage into 

 supposing that Mr. Stoney reproduced in 1856 the instrument previously described 

 by Sir John Herschel. Moreover, the direct reference of objects to the collimating 

 cross was not overlooked by Mr. Stoney, as Sir .John Herchel supposes, inasmuch 

 as no such reference is possible in Mr. Stoney's collimator. 



On a clieap form of Heliostat. By G. Johnstoni; Stoney, M.A., F.B.S. 



This heliostat was planned throughout with a view to cheapness. It costs only 

 five guineas, and yet, in the opinion of the author, who has used the first of them 

 for a year and a half, is quite as ethcient as the more expensive instruments. It 

 has no second reflection, has the adjustments of the mirror under easy control, and 

 is adapted for use at any station within a range of latitude of five or six degrees. 

 It was made for the author by Messrs. Spencer and Son, of Dublin. 



Mr. Stoney expressed the opinion that a heliostat could be made on the same 

 plan at small cost, and yet so large as to be of much use in printing photographs, 

 and especially in enlarging them. 



On the best Forms of Numerical Figures for Scientific Instruments, and a 

 jyroposed Mode of Engraving them. By Lieut.-Colonel A. Stbange, 

 F.E.S., FJi.A.S. 



Mr. G. J. Symon exhibited a Storm Eain-gauge. 



On Self-registering Hygrometers. By E. Vivian, M.A., F.M.S. 



Mean results in meteorology are ordinarily deduced from one or more daily ob- 

 servations at specified houi-s, with corrections for diurnal range ; or from curves 

 traced by photogTaphy or mechanical apparatus. In the rain-gauge, evaporating- 

 vessel, and certain forms of anemometer, the aggregate amomits, however fluctuating, 

 are obtained by accumulative action. 



The latter of these methods is the most certain, and admits of being more readily 

 reduced into a tabular form for the comparison of general averages. At a former 

 Meeting of the British Association the author exhibited self-registering instruments 



