580 REPOET— 1887. 



2. Conduction of Electricity through Gases. 

 By Professor A. Schuster, Ph.D., F.B.S. 



Though a current can usually be sent through a gas only with a high E.M.F.^ 

 yet, if such a current is passing in any part of a vessel containing a gas, a current 

 can be passed, through any other part with an E.M.F. that is only a fraction of a volt.. 



This phenomenon doubtless explains certain electrical actions in the atmo- 

 sphere. 



3. Instruments for Stellar Photography. 5?/ Sir Howard Geubb, F.B.S. 



Referring firstly to the optical arrangements, the author stated that the condi- 

 tion of extent of held, so important and essential in this work, was one which the 

 optician had never before been asked to consider in telescopic objectives; and he 

 described the direction of a series of experiments which he has been carrying out 

 with the object of obtaining an improvement in this particular. These experi- 

 ments are not yet complete, but he was able to state that thej^ appeared to point 

 to the conclusion that a very considerably larger field could be obtained than had 

 been at first anticipated. 



Speaking then with reference to the mechanical arrangements, he described 

 various modifications and additions which it was desirable to make in the equa- 

 torial itself in order that it might best fulfil the conditions required for the new 

 work, and referred specially to the subject of the driving-clock. 



The author considers that it is very desirable to have a well-regulated and con- 

 trolled clock, for although the possession of such does not dispense altogether with 

 the ' eye and hand ' work, it does so to a great extent — sufficient at least to relieve 

 the observer of much of the intense straiii otherwise required. 



By the use of the electric control de'scribed at last year's meeting the author 

 has reduced the maximum error to j^^th of a second ; but this is hardly sufficiently- 

 accurate. 



The greater part of the residual error exists in the screw itself, which, being" 

 necessarily coarse, has not (up to the present) been cut in a micrometer screw- 

 cutting engine, but in an ordinary lathe. 



The author is now engaged in constructing two special machines — one for 

 testing and the other for cutting these screws; and he confidently^ hopes to reduce 

 the errors to ^^th of a second by adopting the following precautions : — 



1 — Increasing radius of sector. 



2. — C'utting screw in special micrometer screw-cutting engine. 



3. — Increasing the delicacy of the ' detector ' of the control by increasing 



its scale. 

 4. — Reducing to the smallest limits the amount of gearing between the 



clock governor and screw. 



The author considers that if the errors be reduced to ^-^th. of a second the 

 accuracy will be sufficient for all practical purposes. 



4. On the Nature of the Photographic Star-Discs and the Bemoval of a 

 Difficiilty in Measurements for Parallax. By Professor C. Pritchabd, 

 D.D., F.B.S. 



The image of a star exposed to a photographic plate driven by a clock having 

 a small rate and subject to small periodic oscillations, as is generally the case with 

 the majority of driving-clocks, is not a simple linear trace, but a series of black dots 

 joined together by intervals less dense. 



This will be the generic form of a star-image when these black dots, &C.,. 

 coalesce, or are superimposed by means of hand-diiving. 



If, for the purposes of measurement for parallax or otherwise, a bright star he 

 covered over by a stop during the greater part of the duration of the exposure of 



