322 



NA TURE 



VFcb. 3, 1887 



useless unless they are prepared almost immediately 

 before they are required, so that as a matter of fact very 

 sensitive plates are now avoided. 



Another limit to the sensitiveness which can be utilised 

 is the diffused light proceeding from the atmosphere, 

 either from the gas of a large town, as in Paris, or from 

 the presence of the moon. Very sensitive plates are 

 liable to be fogged even by diffused ligJit in the case of 

 very long exposures. 



We have before referred to the arrangements employed 

 for enabling the images of stars to be differentiated from 

 any accidental spots or dots on the plate. The plate is 

 practically exposed three times to the region of the 

 heavens, with such a small variation of position, however, 

 that the three images of the star on the plate appear as 

 one to an observer who looks at it casually, and a 

 magnifying glass is really necessary to discover the triple 

 nature of the image. This method of working has been 

 found to have advantages which were not anticipated in 

 the first instance ; thus, for the same total time of ex- 

 posure the images of much more feeble stars are recorded 

 with the three successive exposures than with one alone. 

 This arises from the fact that the stars of the lower mag- 

 nitudes, only being represented by very small points from 

 1/30 to 1/40 of a millimetre in diameter, would escape all 

 observation by the naked eye, and would not be visible at 

 all on paper copies ; while the three exposures give a larger 

 image visible to the naked eye, and perceptible on a paper 

 positive. Moreover, if a small planet is included in the 

 region being photographed, the deformation of the small 

 triangle would instantly betray its presence, even with an 

 exposure of a quarter of an hour. Admiral Mouchez has 

 calculated that a planet at twice the distance of Neptune 

 would be easily recognised in three successive exposures 

 of an hour each, — the motion of Neptune in half an hour 

 quite destroying the triangle which it, Uke the stars, would 

 make were it at rest. 



The real and serious objection to the triple exposure is 

 the wonderful patience and skill that are required to keep 

 the instrument for three consecutive hours, without a 

 nioment's relapse, pointed rigorously towards the same 

 spot in the sky. This is very trying work, and apt to 

 overstrain those who perform it. Admiral Mouchez is 

 alive to the fact that the way to obviate this difficulty is 

 to increase the aperture of the object-glass, and this is 

 what probably will be done before very long. 



Some very interesting information is given regarding 

 the microscopical appearances of the images of the stars 

 seen on the negatives : — "The microscopical study of the 

 cliches presents, moreover, much interest from many 

 points of view, and the appearances of the images of the 

 stars is so characteristic that it is impossible to confound 

 them with accidental spots, as has been generally sup- 

 posed ; were this point of view alone regarded, it would 

 perhaps be useless to multiply the exposures of the same 

 plate. The stars appear on the plate, in fact, not under 

 the simple form of a round spot of uniform black tint 

 diminishing and becoming clearer as the star gets smaller, 

 but as a mass of small, round, black points, very close to- 

 gether towards the centre for stars of the ten or twelve 

 larger magnitudes, and more and more sprinkled, still 

 retammg their blackness, for the fainter stars ; and at tlie 

 extreme limit beyond those stars which give a definite 

 and certain image, there still appear on the cliche some 

 small groups of little points scattered sparsely, but evi- 

 dently recording still fainter stars, the existence of which 

 can only be suspected without any means of further con- 

 firmation. 



" Unfortunately, whatever progress we may make in 

 optics or in photography whatever, penetrating and 

 sensitive power we may hope to give to our instruments, it 

 is evident that we shall never succeed in seeing the most 

 distant stars, and that at whatever limit we may arrive, 

 there will always be beyond it an infinity of others lost in 



the profundity of the heavens which will always escape 

 our knowledge, but it is by photography and the scientific 

 study of negatives that we shall be able to go further than 

 by any other means. From a chemical point of view also 

 the microscopical examination of the stellar images will 

 not be without interest, because it will help us to under- 

 stand how the light acts upon the molecules of the in- 

 soluble salts of silver which are contained in the stratum 

 of organic material which forms the sensitised plate. It 

 is not, as I have already stated, in giving a uniform tint, 

 more or less decided, according to the magnitude of the 

 star, over the whole image, but really in decomposing 

 a greater or less number of particles of salts of silver 

 over this area, that the light works ; so that we can 

 define the image of a very feeble star as a resolvable 

 nebula, and the others as insolvable nebukt surrounded by 

 a resolvable portion. I have never seen around any of 

 these images the rings referred to by several astronomers, 

 which have the appearance of diflraction rings seen in 

 telescopes. 



" To establish the relationship between the scales of the 

 optic and photographic magnitude of the stars. Bond has 

 made a series of interesting experiments by varying the 

 time of exposure and the aperture of the object-glass. 

 These experiments have led him to an interesting result 

 on the mode of action of light. He has found that a 

 certain time elapsed before the action manifested itself at 

 all, and then that it did so suddenly, ten or a dozen mole- 

 cules of salts of silver in each superficial second of arc 

 were attacked by the light ; after this the number increased 

 very rapidly according to the time of exposure. This 

 mode of action seemed to him obscure and difficult to 

 explain. But it seems to follow from these facts, and 

 from the examination of our cliches^ that in the manufac- 

 ture of the bromide of silver, and the preparation of 

 sensitive plates, it is of the highest importance to obtain 

 the finest possible pulverisation of the salt." 



As there is to be a Conference of Astronomers at Paris 

 next Easter to discuss the whole question of astronomical 

 photography, it is well that Admiral Mouchez and his 

 staff are accumulating so many facts to help in the 

 discussion. 



METEOROLOGICAL CONDITIONS AT THE 

 TIME OF THE ERUPTION OF MOUNT 

 TAR A WERA, NEW ZEALAND 



IN the Government Sanatorium at Rotorua there is a 

 self-registering barometer kept by Dr. Cinders. This 

 shows that at 9 a.m. on June 9, the atmospheric pressure 

 was 29'30 (at about I coo feet above the sea). 1 1 decreased 

 and reached its lowest point of 2900 at 4 p.m. on the 9th. 

 It then began to rise. At midnight it was 29 oS, and at 

 I a.m. on the loth — just before the eruption — it was 29'io. 

 This pressure was maintained all through the principal 

 part of the eruption, after which the glass began to rise 

 again, reaching 29'25 at noon on the loth. The curve, 

 elsewhere smooth and even, shows from 3 30 a.m. to 6 a.m. 

 a number of small oscillations which treble its thickness. 

 None of these oscillations are recorded before and none 

 after 6 a.m. on the 9th, except a single one at 5 p.m. on 

 Friday, the nth. These oscillations are attributed to 

 earthquakes, but, whatever may have been their cause, 

 they certainly mark the outburst of Rotomahana and the 

 crisis of the eruption. 



Another barometer at Ohinemutu, belonging to Mr. 

 Edwards, of the Native Lands Court, read as follows :— 



June 9, 10 a.m ag'soinches 



,, 4.3op.m 29'oo ,, 



,, 10, 1.55 a.m 29-20 ,, 



The following is the rainfall at Kotorua : — 



June 4 I '25 inches 



„ 5 °'58 .. 



