SCIENTIFIC.

Otago Witness, Issue 956, 16 May 1889, Page 35

SCIENTIFIC.

PHOTOGRAPHING THE UNSEEN.

MR ISAAC ROBERTS' PICTURES.

. It is now some three years since attention [ was called in these pages to a new and momentuous departure in the science of astronomical photography. MM. Paul and Prosper Henry, of the Paris Observatory, had just announced that by means of the photographic camera they had obtained pictures of celestial objects which had never been seen by the eye of man, and in all probability would always remain inaccessible to human vision. They had obtained an impression on the gelatine plate of a new nebula in the Pleiades which they were quite unable to see with the telescope^ and in the constellation Cygnus they had obtained pictures of whole hosts of stars which the human eye, assisted by the most powerful telescopej was quite unable to detect. Since the year of these memorable triumphs of photography, the camera astronomers hare by no means been idle. The successes of an Englishman in the art of photographing the unseen have now to be recorded. Mr Isaac Roberts, F.R.A.S., F.G.S., of Liverpool, 1 has eVen distanced the achievements of MM. HSnry. An ardent and able amateur astronomer, Mr Roberts has, with his own resources, eclipsed the work of our best official observatories, and indeed of all other rivals ; ( and his photographs have furnished the chief record of the English astronomical calendar of the winter of 1888-9: ,

At the December meeting of the Royal Astronomical Society Mr E. Knobel called attention to a photograph of some stars in Cyngus just taken by Mr Roberts. The photograph dealt with the same area of the heavens as that taken by MM. Henry at Paris in. 1885. . The Paris photograph recorded 3000 stars; but Mr Roberts' photograph, taken in the following year, showed 6000 stars, whilst his second photograph of' the same area, taken in 1887 under more favourable conditions, showed the astonishing number of 16,206 stars. Thus in the same sky area there were discovered 13,000 more stars than appeared in MM. Henry's photograph.

Another photograph exhibited at the same meeting by the same astronomer is remarkable in another direction, and picturesque beyond description ; it is a photograph of the beautiful nebula in Andromeda (31 Messier), which, even as rudely presented in ordinary astronomical text-books, with its characteristic dark rift, always commands our admiration. As shown in Mr Roberts' photograph, which was obtained by two hours' exposure, the nebula is now seen for the first time in intelligible form. Apparently a new solar system is in process of condensation from the nebula, which is obviously revolving. The central sun is seen in the midst of nebulous ma&ter, which in time would be either absorbed or separated into rings more less symmetrical with the nucleus, and presenting a general resemblance to the rings of- Saturn. The two smaller nebulas, Herschel 44 andHerschel 51, seem as though they were already undergoing their transformation into planets. No higher testimony to the value of these photographs and the merits of Mr Roberts as an observer could be given thanj;of Mr A. A. Common, whose photograph of the great nebula, in Orion, has given him ample experience in. the same enormously difficult field of astronomical work. The testimony of Mr Norman Lockyef and Mr Ranyard was no less favourable and unstinted. An engraving of the new photograph of the Andromeda nebula appears in the " Obsrvatory " for February last, and gives some notion of the advance which has been made in the portrayal of this wonderful object. Now that our leaders in astronomical photography have overcome the great mechanical difficulty of keeping a large telescope, several tons in weight, moving accurately as the earth moves, for three hours together, and in precise: apposition with the star or nebula under observation (if indeed they have not evaded this onerous task by an astute arrangement for moving the photographic plate alone), it may be considered that astronomical photography is making marvellous progress through a precocious infancy, and that we shall speedily hear more of its triumphs.—" Leisure Hour!"

—Why does iron feel colder than wood when both are subjected to the same temperature? Substances of the same temperature' feel hot or cold to the touch according to their conductivity. Iron conducts heat about 300' times more easily than wood does. Therefore, on touching cold iron with the naked hand, 300 times more heat passes out of the warm hand than if wood were being touched. If the iron is hotter than the hand, 300 times more heat is communicated to the hand than by wood at the same temperature." In the first case, the iron seems colder than the wood ; in • the second, it seems hotter. For the same reason, a burning piece of wood can be held close to the burning part with the naked hand, whilst a piece of iron heated at one end can only be held at a distance from that end.