August 29, 



1S89] 



NATURE 



417 



Electrolysis of Potassium Iodide. 



The following form of this expeiitnent differs from that 

 producing blue iodide of starch usually given in text books, and 

 as I am not aware of it being known it may be of interest to 

 those engaged in teaching the subject to which it belongs. 



Into a Utube pour a solution of the salt coloured with red 

 (slightly acid) litmus solution, then, on introducing the electrodes 

 (carbon for the anode) and passing a current, the solution in the 

 anode limb is turned brown, due to the solution of I in KI, 

 while at the kathode the liquid turns blue from the liberation of 

 potassium. 



This form of the experiment is very pretty, and is suitable for 

 large classes. 



For small classes the poles of a battery may be drawn over 

 red litmus paper moistened with KI solution. 



It is well to exhibit the action of iodine and potassium (potash) 

 upon separate portions of the red solution before conducting the 

 experiment. E. F. MoNDY. 



Dacca College, Bengal, July 24. 



Spherical Eggs. 



Will you allow me to thank Profs. Greenhill and Liveing for 

 their notes in reply to mine on the question of the packing of 

 spherical eggs. 1 he chief drawback of life in New Zealand is 

 the inaccessibility of works of reference, such as those mentioned 

 by Prof Greenhill. W. Steadman Aldis. 



Auckland, New Zealand, July 11. 



ANOTHER PHOTOGRAPHIC SURVEY OF THE 



HE A VENS. 

 A^ZE gather from two circulars received from Prof. 

 * * Pickering that another photographic map of the 

 heavens is to be made, in addition to that arranged for 

 at the Paris Conferences. The first circular runs as 

 follows : — 



It is proposed to establish an Astronomical Observa- 

 tory on one of the mountains of Southern California, 

 under climatic conditions probably superior to those at 

 any similar institution now existing, with possibly one or 

 two exceptions. It is therefore extremely important to 

 increase these natural advantages by a plan of work and 

 form of instrument which shall give results such as can- 

 not be obtained elsewhere. Moreover, in California the 

 interest in astronoiny is wide- spread. There are many 

 persons of large means who might be willing to make an 

 important contribution to science if they could be sure 

 that the promised results would be attained. The plan 

 detailed below provides for a telescope with which stars 

 could be studied that would be beyond the reach of any 

 other instrument. The amount of material accumulated 

 would be far greater than that obtained by telescopes of 

 the usual form. A satisfactory test of the work could be 

 made before a large part of the money had been expended. 

 No great delay would probably arise, so that the donor 

 could soon see the results arising from his gift. When 

 money is given to erect a building without sufficiently 

 endowing it, the value of the gift is greatly diminished if 

 the name of the donor is attached to the building. This 

 objection does not hold in the present case, since a large 

 part of the expenditure is for current work. On the other 

 hand, the donor's name would always be honourably 

 associated, not only with the instrument, but with the 

 work done with it. As in the case of the Henry Draper 

 Memorial, it is believed that by such a living reminder 

 a patron of science will be more widely known and 

 appreciated than by a much larger expenditure for a 

 building or fund. 



Photography is rapidly changing the older methods of 

 astronomical study. It gives an accurate representation 

 of many objects at the same time ; and, since copies may 

 easily be made, it permits the results to be studied at 

 leisure at any place and time. In a recent paper before 

 the National Academy of Sciences, the writer recom- 



mended the construction of a large photographic tele- 

 scope of the form described below. The lens should be 

 like that used by photographers rather than like that of 

 an astronomical telescope, and should consist of two 

 achromatic lenses. Its aperture should be 24 inches, and 

 its focal length 1 1 feet, thus giving images of objects on a 

 scale of I millimetre to a minute of arc. Its great 

 advantages would be the large region covered by a single 

 photograph, since 5° square could be represented by it 

 upon a plate 12 inches square. This is six or eight times 

 the area covered by a telescopic objective. The time 

 required to photograph a given portion of the sky would 

 be reduced in this proportion. Such a lens, if mounted 

 in a favourable location and kept constantly at work, 

 would add more to our knowledge of the stars than could 

 be obtamed by a large number of telescopes of the usual 

 kind. A telescope of this form, but of one-third its size, 

 is now in use at Cambridge, and illustrates the advantages 

 and amount of work which can be obtained by such an 

 instrument. 3186 photographs have been taken with it, 

 and from them a catalogue of 28,000 spectra of 11,000 

 stars has been prepared; also a catalogue of 1200 

 stars near the equator as standards of brightness, 

 and a catalogue of 1000 stars within 1° of the Pole, 

 where the most extensive existing catalogues only con- 

 tain about forty stars. A search for new nebulae was 

 made on a small number of these plates covering 

 about 1/250 of the entire sky. Eighteen nebulae were 

 already known to exist in this region. Twelve new 

 ones were found upon the plates. These results are 

 derived from a small portion of the entire series of plates, 

 and much additional material will be extracted from 

 them. The large telescope should first be used in making 

 a map of the entire sky. All the stars north of — 30"^ can 

 be advantageously photographed in the latitude of 

 Southern California. This region, covering three-quarters 

 of the sky, has an area of 30,000 square degrees. If each 

 plate covered 25 square degrees, 1200 plates would be 

 required. There are about 3600 hours of darkness in a 

 year. Allowing one half for clouds and moonlight, and 

 one-third of the remainder for imperfect plates, the whole 

 work could be done in one year, allowing an exposure of 

 an hour to each plate. Perhaps longer exposures would 

 be found advisable, and two years should be assigned to 

 this work. An equal time should be spent in repeating 

 the work, since it is essential that every part of the sky 

 should appear on at least two plates, to verify all sup-, 

 posed discoveries. By using a prism to cover the lens 

 the spectra of all the stars may be taken in the same way 

 and in the same time. When this work is completed, it 

 should be repeated, since we ought to have a complete 

 map of the sky at intervals of about five or ten years in 

 order to detect changes. Moreover, the improvement in 

 photographic processes would perhaps be so great that a 

 second series of plates would be desirable. The recent 

 applications of erythrosin and other coal-tar products to 

 photographic plates render them much more sensitive to 

 red and yellow light. The difficulty of photographing 

 satellites, asteroids, comets, nebula;, and red variable 

 stars may therefore be diminished. 



The telescope should be mounted in a place having 

 the best possible climatic conditions, preferably on a 

 mountain where the air is as clear and steady as possible. 

 No work need be done there except taking the photo- 

 graphs and developing them. Accordingly, the work might 

 be done by a single observer ; but, to avoid interruptions 

 due to illness or accident, at least two would be required. 

 Since their duties would be a routine requiring only 

 ordinary skill, such services would not be expensive. 



The results would take the form of a series of glass- 

 photographs about a foot square, each of which would 

 depict all the stars visible in a part of the sky 5° square. 

 Glass positives could be made from these by direct 

 printing, and copies could be furnished for about a dollar 



