REPOKT OF COMMITTEE ON OBSERVATORIES 133 



details of the light swing-frames with their counterweights carrying 

 the eye piece at each end of the telescope and readily turned aside 

 when not in use. I feel assured that such an instrument in the 

 hands of a skillful observer would yield zenith distances of an accu- 

 racy not yet attained. One of our first opticians assured me that 

 by the use of twin discs of glass and alternate grinding there would 

 be no difficulty in producing the requisite pair of object glasses. 



The transit instrument should be reversible on every object. It 

 should have a clean drawn cylindrical tube of mild steel, attached 

 in the simplest efficient manner to the middle of the enlarged steel 

 axis through which it passes. This kind of tube was suggested by 

 Sir David Gill in conversation many years ago. The transit to be 

 taken by means of a movable recording wire — say, 10 seconds in 

 each position of the instrument — by the well known method used so 

 largely by Professor Albrecht and his staff in Germany. By this 

 method collimation error and inequality of pivots are at once elim- 

 inated, and, as it seems to me, the troublesome magnitude equation 

 is practically evaded. Moreover, it is certain that the personal 

 equation is confined within extremely narrow limits. As it would 

 be necessary to observe the sun with this instrument, it would prob- 

 ably be desirable to use a reversing eye piece ; but experience would 

 doubtless soon show whether this is desirable or not. 



Group b. — This work — the precise observation of the 5,400 re- 

 maining stars brighter than the seventh magnitude between — 20 

 and the south pole — could probably be rapidly and efficiently ac- 

 complished by the use of a meridian circle, which I should like to 

 see made of steel, with the graduations on gold, platinum, or on an 

 alloy of gold and palladium ; but by no means upon silver, which is 

 so liable to tarnish and necessitate risky cleaning. 



The observations in class 4 are well worth undertaking with the 

 least practicable delay, as they will gain largely in value by every 

 year that elapses after they are once secured. By carefully boxing 

 in the circle of the instrument used and securing an efficient circu- 

 lation of the air confined within the box, it is probable that the 

 accuracy of this class of observations could be measurably increased. 

 Class 2 seems to me to be the least important part of the proposed 

 undertakings, the resulting stellar distances being apparently pecu- 

 liarly mixed up with those of the few available comparison stars. 

 But may we not hope, now that the displacement of the earth's axis 

 of rotation with regard to the observatory can be taken into account, 

 that the fundamental observations of class 5 may begin to indicate 



