British Association for the Advancement of Science. 285 



or A/' in diameter. Many of them occm- in crowded parts of the 

 milky way, with not fewer than 80 or 100 stars in the field, of 

 view at once. The drawings are copies of much more elaborate 

 originals, and merely selected from a greater collection, illustra- 

 ting three of the most singularly constituted nebulse in the S. 

 Hemisphere, viz. 6 Orionis, »? Argus and. 30 Doradus. Sir J. ex- 

 plained how, by means of a small achromatic collimator placed 

 inside his great sweeping telescope, he was able to obtain nearly 

 the same precision as was to be had in fixed observations ; al- 

 though from the ropes and wooden frame with which it was 

 mounted, it was subjected to great hygrometric and pyrometric 

 changes of form and position. These changes, by affecting alike 

 the cross of the collimator, and the object, were readily detected 

 and corrected. — Dr. Robinson^ spoke in praise of the accuracy of 

 the positions given in Sir J. Herschel's catalogues ; and in favor 

 of the application of reflecting telescopes to divided instruments. 

 Notwithstanding the great increase in late years, of the size of 

 achromatics, it seemed improbable that they would ever reach a 

 magnitude which could not be easily overmatched by reflection. 

 Something to this effect had been done in Ireland. In his own 

 observatory was a reflector of 15 inches aperture, applied to an 

 equatorial of cast iron, which gave polar distances with a proba- 

 ble error of about 6 seconds, and right ascensions to the ultimate 

 reading of the hour circle verniers. The artist who executed this, 

 had since made a reflecting transit of six inches aperture, which 

 performed well, and its collimator was not affected by reversion. 

 Sir J. Herschel remarked that the only change in a nebula, 

 which he had yet noticed, was in that of Orion. A small trans- 

 verse strip, which, when he first figured that nebula, was straight, 

 had become curved, and showed a knotty appearance, which cer- 

 tainly it did not possess before. 



Remarkable Phenomena of Halleifs Comet. — Sir J. Herschel 

 related the following. One of the most interesting series of ob- 

 servations, I had to make at the Cape of Good Hope, was that of 

 Halley's Comet. This comet is the great glory of modern calcu- 

 lation. To see the predicted return of such a body now verified 

 for the second time, true to a single day, — nay, to a i&w hours — 

 of his appointed time, after an absence of 75 or 76 years, during 

 which it has been subjected to the unceasing perturbations of all 

 the planets, and especially persecuted by Jupiter and Saturn, 



