566 Royal Astronomical Society. 



cumulated a body of probability in favour of the resulting parallax, 

 which cannot but impress every reasonable mind with a strong de- 

 gree of belief and conviction. 



Now, all these circumstances have been found by M. Bessel, in 

 his discussion of the measures taken by him (which have been very 

 carefully and rigorously examined by Mr. Main in the paper al- 

 luded to, as have also M. Bessel's formulae and calculations, for in such 

 matters nothing must remain unverified), to prevail in a very signal 

 and satisfactory manner. Not one case of discordance, in so many 

 independent particulars, has been found to subsist; and this, of itself, 

 is high ground of probability. But we may go much further. Mr. 

 Main has projected graphically the deviations of the distances (a c) 

 and (b c) from their mean quantities (after clearing them of the ef- 

 fects of proper motion and of the minute differences of aberration, 

 &c). Taking the time for an abscissa, and laying down the devia- 

 tions in the distances so cleared as ordinates, two curves are obtained, 

 the one for the star (a), the other for the star (6). Each of these 

 curves ought alternately to lie for half a year above, and for half a 

 year below, its axis. — It does so. Each of them ought to intersect 

 its axis at those dates when the maximum and minimum of the 

 other above and below the axis occurs. With only a slight degree 

 of hesitation at one crossing — it does so. The points of intersection 

 with the axis ought to occur at dates in like manner calculated 

 a priori ; and so they do within very negligible limits of error. And, 

 lastly, the general forms, magnitudes, and flexures of the curves 

 ought to be identical with those of curves similarly projected, by 

 calculation on an assumed resulting parallactic coefficient. This 

 is the final and severe test : Mr. Main has applied it, and the results 

 have been placed before you : — oculis subjecta fidelibus. If all this 

 does not carry conviction along with it, it seems difficult to say what 

 ought to do so. 



The only thing that can possibly be cavilled at is the shortness of 

 the period embraced by the observations; viz. from August 1837 

 to the end of March 1840. But this interval admits of five inter- 

 sections of each curve with its axis ; of two maxima and two minima 

 in its excursions on either side; and of ample room for trying its 

 agreement in general form with the true parallactic curves. Under 

 such circumstances, it is quite out of the question to declare the 

 whole phenomenon an accident or an illusion. Something has as- 

 suredly been discovered, and if that something be not parallax, we 

 are altogether at fault, and know not what other cause to ascribe 

 it to. 



The instrument with which Bessel made these most remarkable 

 observations is a heliometer of large dimensions, and with an ex- 

 quisite object-glass by Fraunhofer. I well remember to have seen 

 this object-glass at Munich before it was cut, and to have been not 

 a little amazed at the boldness of the maker who would devote a 

 glass, which at that time would have been considered in England 

 almost invaluable, to so hazardous an operation. Little did I then 

 imagine the noble purpose it was destined to accomplish. By the 



