January 26, 1899] 



NA TURE 



295 



them surpasses all the others, and has an intensity which ue may 

 •call I. It is situated in the direction A. The next maxima, 

 one on either side of the principal one, have intensity I'/g, where 

 171 = 4/t- ; the next pair beyond these have intensity I725 ; 

 and so on — the intensities of the lateral maxima decreasing in- 

 versely as the squares of the odd numbers. These are conse- 

 quences of the theory of diflfrartion gratings. From the above 

 law it follows that the maxima soon become much too faint to be 

 seen by the eye, or to affect a photographic plate, so that what 

 alone can be made visible is the group in the immediate neigh- 

 bourhood of the position A. This group is what seems to be 

 represented on a very large scale in some of Prof. Michelson's 

 figures, but which in our spectroscopes is so compressed laterally 

 that the whole group appears like one line. 



A similar group is necessarily substituted for each physically 

 single line wherever there is one of those limitations of aperture 

 which must occur within the interferometer itself, and, in fact, 

 in every optical instrument. In the astronomical telescope it 

 t'Ccasions the spurious disc of a star with its attendant rings. 

 The only question is whether the intervals between the maxima 

 of the group are sufficiently wide to be detected by the inter- 

 ferometer ; and as this is simply a question of aperture, or, in 

 the case of a grating, of the number of rulings, it follows that 

 these instrumental groups may be distinguished from physically 

 real groups by the circumstance that the maxima present in them 

 will become more widely spaced if the aperture (or number of 

 rulings) is reduced. 



II. An apparently doitble line. — Where the apparatus em- 

 ployed limits the aperture at only one place, the group, which 

 will be substituted for a physically single line, is that de- 

 scribed above ; at least, if the instrument is in adjustment. 

 But if the arrangements are such as to introduce an actual or 

 virtual limitation of apertures at two places, a double line may 

 result, accompanied by faint appendage lines. What then 

 occurs was discussed in a communication made by the writer to 

 the British Association in 1894 (see Report of the Oxford meet- 

 ing, p. 5S3 ; when also were exhibited examples of such spurious 

 double lines coarse enough to be seen without an interferometer, 

 and where the appendage lines predicted by theory were also 

 seen when the light was intense. 



The foregoing instrumental effects arise when the apparatus 

 is accurately coUimated. If it be much out of adjustment the 

 groups, substituted for a really single line, are more complicated. 

 But whatever they may be, it is certain that if the resolving 

 power of the interferometer can go sufficiently far, it will bring 

 them into view if they are sufficiently bright to be seen, and that 

 we must then be on our guard against mistaking a resolution of 

 this kind for evidence that the line under examination is 

 physically multiple. G. Johnstone Stonev. 



S Upper Hornsey Rise, N , January 13. 



The remarks put forward by Dr. Stoney are the well-known 

 results of the wave theory, and Prof. Michelson, no doubt, is 

 quite familiar with all that has been said in this department. In 

 addition, he alone is quite familiar with what it is that is actually 

 observed in his interferometer, and therefore must be in a posi- 

 tion to give a strong opinion as to whether the observed effects 

 are spurious and due to dilTraction or not. 



It was with this feeling that I w rote my remarks (p. 228), and 

 I wished to draw some further information from Prof. Michelson 

 on this important subject, rather than in any way to cast doubt 

 on the correctness of his conclusions. I have never worked 

 with the interferometer, and therefore cannot pronounce upon 

 its performance as an optical instrument, but I trust Dr. Stoney's 

 interesting and important observations may have the effect of 

 drawing a full explanation of all the outstanding difficulties. 



Bardowie, Orwell Park, Dublin. Thomas Presion. 



A Note on Catching Insects, and the Behaviour of the 

 Bulldog-Ant of South Australia. 



DuRixr, my visit to South Australia, I wished to obtain some 

 specimens of the insects of the country, for my naturalist friends 

 at home. At first I experienced considerable difficulty in catch- 

 ing those whose movements were rapid, without injuring their 

 bodies. Recently I have been able to secure nearly every 

 specimen seen, by the following method. 



.\ small antitoxin syringe was charged with benzol, and a 

 small jet of the liquid was directed towards the beast sought for 



NO. 1526, VOL. 59] 



(a large tarantula, for example) ; the result of this form of 

 attack was to render the beast almost instantly inert, so that it 

 was easily secured. I am not at all sure that benzol is the best 

 liquid for the purpose ; but I used it, as it happened to be the 

 only substance I could obtain, at a distance from a township, 

 which appeared likely to produce the desired effect. I find that 

 insects whicli could be easily captured a month ago when it was 

 fairly cool, have become wonderfully active now that the sun- 

 shine temperature is about 150 to 155° F. 



I placed a large bulldog-ant, about one inch in length, in a 

 glass bottle three and a half inches in diameter ; I noticed that 

 when the bottle was on its side, and the ant was clinging to the 

 upper surface of the bottle, with its back pointing downwards, 

 that if the ant was detached by a slight jerk, it fell on its feet. 

 I repeated this experiment many times, with seven different 

 bulldog-ants ; in every case the ant fell feet downwards, after 

 the well-known manner of a cat. 



As I am no naturalist, I maybe describing an acrobatic move- 

 ment of the bulldog-ant already well known to students of 

 natural history. But even shotdd this be so, probably the fact I 

 have mentioned may be of interest to others. 



Fredic. J. jER\'is-SMrrH. 



(luanbi, Mt. Barker, South Australia, December 16, 1898. 



A New Dome for Equatorials. 



With reference to the note in Nature of January 12 (p. 

 257), on " A [New Dome lor Equatorials," allow me to point 

 out that a rotary dome without a shutter was, as far as I know, 

 first constructed for the observatory in Strassburg. The late 

 Prof. Winnecke, at the meeting of astronomers in Berlin on 

 September 5, 1879, referred to it as follows ( Vierleljahrsschrift 

 aer Astronoinischen Gesetlschaft^ \\ Jakrgang^ p. 334): "In 

 consequence of the construction of the altazimuth, in which the 

 telescope was placed at the end of the axis, the dome of the 

 north tower had to have an unusually large aperture. This 

 was easily accomplished by employing two separate halves of 

 a hemisphere, which could be rolled back on wheels situated 

 above the circular rail, l)y which means a complete aperture of 

 2h metres could be obtained. The mechanism for the movable 

 parts works easily and surely." 



The same construction is described in the Annalen der Kais. 

 Univ. Slernwarte in Slrasshiirg, vol. i. p. 7 ; an illustration 

 of this will be found on Plate vii. (the dome to the left) : al- 

 though both domes — that to the right holding a small refractor, 

 viz. a comet-seeker — are represented as closed, the difference 

 can be seen at once. 



I trust that this short historical remark may be mentioned in 

 the next number of Natit RE. E. Becker. 



Kais. Univ. Sternwarte, Strassburg, i. E., January 15. 



Luminosity of Sugar. 



The communication of Mr. J. Burke to the British .\ssocia- 

 tion, on the luminosity of sugar (Nature, vol. Iviii. p. 533) 

 recalls to my mind an illustration on the large scale of the same 

 interesting phenomenon. 



In the process of making what is known as granulated sugar, 

 ordinary refined sugar is passed through a revolving sheet-iron 

 cylinder, where it meets with a current of hot air, and is warmed 

 and dried at the same time. On leaving the drying cylinder the 

 sugar is taken by a travelling band, or other carrier, and dropped 

 into a hopper, from whence it is drawn for packing. The sugar 

 falling into the hopper soon forms a pyramidal heap, and 

 when this is examined in darkness, the top of the heap, where the 

 stream of sugar lands, is seen to glow with a steady light blue 

 luminosity. 



On rubbing tw-o pieces of ordinary lump or t.ablet sugar 

 together, in darkness, the glow is readily produced, and when 

 a piece is broken in two there is a bright flash at the moment of 

 fracture. The light produced in this manner is not affected by 

 moistening the lumps with alcohol, but is intensified when water 

 is used. 



When hard lumps of sugar are crushed by striking with any 

 solid body, such as a shovel, the glow appears as a flash, and is 

 easily seen even in the presence of dull light, but is, of course, 

 best viewed in complete darkness. Even a heap of loose sugar 

 responds to a blow from a shovel. 



Sometimes sugar, as purchased retail, when left in the bag 

 during dry weather, sets to a hard mass. On scraping such with 



