Mr. Pond on ih-e Parallax of a. Lyra. 295 



I can show however in the present as in tlie former process, 

 that no error from temperature, affecting the instrument, has 

 mtroduced itself into this series of observations ; for I obtain 

 the same resuh from the readings with two microscopes as 

 from those made with six. 



In the case of two microscopes, the angular distance is 

 measured upon two arcs only. Now it cannot be for a mo- 

 ment contended that an error from temperature, so great as 

 not to be corrected by six microscopes, will not be much ex- 

 aggerated by employing only two. The errors then, if any, 

 must arise fi-om the effects of temperature on refraction, and 

 not from the changes it occasions in the instrument. But 

 from the season which I have chosen for this investigation, 

 and from the care that has been taken to equalize the tempera- 

 ture, the errors arising from the latter cause must be almost 

 insensible. My observations, thus conducted, indicate in the 

 most decided manner, that the parallax of « Lyrae cannot ex- 

 ceed a very small fraction of a second. The advantages and 

 disadvantages of the Dublin and Greenwich methods are in 

 this process much more nearly balanced than in the former. 

 The Dublin instrument has the great advantage of determin- 

 ing the zeiiith distance in the course of a few minutes; whereas 

 at Greenwich twentj'-four hours at least, and frequently seve- 

 ral days elapse, before a complete observation of the double 

 altitude can be obtained by the method of reflection. This 

 disadvantage attending the Greenwich method could only be 

 remedied by employing two mural circles for observing a star 

 on the same night, both by direct vision and by reflection. 



I have now to consider that argument on which the greatest 

 reliance in favour of parallax has been placed, namely, that 

 founded on the actual determination of the solar equation from 

 the observations made with the Dublin mstrument. 



This argument may, I think, be thus stated. By a series 

 of observations made with a given instrument two equations 

 have been disengaged, previously considered as unknown in 

 amount, but known only as to the law of their variation. Of 

 these, one is much smaller than the other. Hence it is inferred, 

 that as the instrument has faithfully disengaged the smaller 

 equation (respecting which there is* no dispute), it must be 

 admitted with equal fidelity to have disengaged the larger, 

 which might be su])posed the easier operation of the two. 

 This reasoning is strictly logical, as proving the disengage- 

 nicjit of two ofjuations; but it by no means proves the larger 

 e(|uation to be caused by parallax. The larger equation here 

 t<) be disengaged is after ull so small, that it is impossible, in 

 dilK-reiit points of its })eriod, to show that the law assumed 



coincides 



