344 



NA TURE 



[February 13, 1908 



H-S Group. — The elements of this group when subject 

 to a very soft X-radiation emit an almost perfectly 

 scattered ra:diation of intensity proportional to the mass 

 of radiating substance traversed by a beam of definite 

 intensity. 



This conclusion has been based on observations of 

 absorbability, ionising power, polarisation, and distribu- 

 tion of intensity of the secondary rays. 



With a moderately penetrating primary the scattering 

 becomes less perfect — as shown by the above tests — and 

 with a very penetrating primary beam there is con- 

 siderable difference in character between the secondary 

 and primary rays. This is not due to a superposition 

 of a second radiation on the purely scattered ; the 

 purely scattered disappears, and in its place is a radiation 

 more absorbable than the primary producing it, one which 

 is less completely polarised, possesses greater ionising 

 power, and is distributed in a way which e.\hibits less 

 perfect control of the primary pulses over the radiating 

 electrons. 



Cr-Zn Group. — A striking characteristic of this group 

 is the enormous ionisation produced by the secondary 

 beams — of the order of loo times that produced by an 

 equal mass of one of the (H-S) group. This radiation, 

 though produced by a heterogeneous primary, consists 

 almost entirely of homogeneous rays of a very absorbable 

 type. For certain primary beams there appears to be a 

 more or less perfectly scattered radiation (producing i per 

 cent, or 2 per cent, of the total ionisation) mi.xed with this. 



Some remarkable properties of these homogeneous radia- 

 tions have been referred to in a paper published by us. 

 Their intensity is for large ranges in the penetrating power 

 of the primary proportional to the ionisation produced by 

 the primary beam in a thin film of air, so that it is highly 

 probable that the radiation is produced during the process 

 of ionisation in the radiating substance. 



This radiation has not been found to disappear with any 

 changes we have made in the primary beam. 



.4^-7 Group. — This group is characterised • by its 

 sensitiveness to changes in the character of the primary 

 rays when they are of ordinary penetrating power. 

 Though for moderately penetrating primary beams the 

 secondary, rays are heterogeneous and do not differ in 

 penetrating power very considerably from the primary, 

 they do not . exhibit the polarisation effect and are not 

 distributed- in the manner showing scattering as exhibited 

 by rays from the H-S group. These rays are not super- 

 posed on a. radiation which could be classified with that 

 from the H-S group. We have recently found, however, 

 that some of the group (if not all) may be reduced to 

 order by using a very easily absorbed primary. In this 

 case there appears by every test made to be almost perfect 

 scattering. The intensity, however, appears to be several 

 times- as great as that from an equal mass of a substance 

 in the H-.S group. 

 , W—Bi Group. — These substances much more closelv 

 resemble the Cr-Zn group in the intensity and constancy 

 in character of the rays they emit. They have, however, 

 not yet been examined carefully. 



Substances with atomic weights between those in the 

 above-mentioned groups possess some of the characteristics 

 of the two groups ijelween which they lie. It appears 

 possible that similarity in behaviour of all the different 

 groups will be discovered by great variation in the pene- 

 trating power of the primary beams — one group when 

 subject to a certain kind of primary beam behaving as 

 another group when subject to' a primary of different 

 penetrating power. 



The above grouping has, however, been based on the 

 behaviour when subject to beams of ordinary penetrating 

 powers, and the generalisations hold with very few excep- 

 tions. C. G. B,\RKLA. 



C. A. S,\DLER. 



University of Liverpool, January 31. 



Auroral Characteristics of Clouds. 

 The question has often been raised as to a possible 

 connection between the aurora and the formation of clouds 

 in the upper atmosphere. Observers in high latitudes have 

 described cases in which, after a bright aurora, clouds 

 have retained in some measure the' forms of the previous 

 NO. 1998, VOL. 77] 



light display. In other cases high clouds have been 

 observed to arrange themselves in a formation very similar 

 to the arch and streamers which are so characteristic ot 

 the aurora. 



It may be of interest to those who have studied this 

 question to record an exhibition of the latter kind which 

 was observed here yesterday evening. During the greater 

 part of the day the sky had been overcast with alto- 

 stratus clouds, which, as usual, began to dissipate soon 

 after sunset. Between ten and eleven o'clock (local time) 

 the greater part of the sky was clear, but there remained 

 in the south a mass of light cloud which formed an arch, 

 from which bands spread out in all directions as if radia- 

 ting from a point on the horizon under the middle of the 

 arch. The effect was remarkably like an aurora except 

 that there was no rapid motion of the bands or streamers. 

 So far as I could judge from the Pole Star, the centre of 

 the arch was due south, and its top about 5° above the 

 horizon, but being on the river at the time in a native 

 boat I had no means of making accurate measurements. 

 Small masses of cloud in other parts of the sky exhibited 

 wave lines, but they did not appear to be parallel to the 

 main streamers, nor did the latter look like a wave forma- 

 tion. In about half an hour the outlines had lost their 

 sharpness, and the whole effect became that of a light, 

 diffuse mass of cloud. 



The interest in this observation is two-fold. Those who 

 hold that the clouds whicli take upon themselves the form 

 of the aurora are due to the same causes as the aurora 

 will be interested to know that this formation can be 

 seen within 16° of the equator, and also that the centrc- 

 of the arch appeared to be due south. Those, on the 

 other hand, who hold that there is no connection betweei> 

 the two phenomena will feel their position strengthened 

 in that an almost exact imitation of the aurora has been 

 seen in the clouds in a region which is supposed to be 

 almost entirely free from the aurora, thus lending support 

 to the idea that such a formation is only one of the 

 infinite number of possible cloud formations. 



George C. Simpson. 



Moulmein, Burma, January 13. 



Reissner's Fibre in the Fro?. 



Is October last, at the suggestion of Prof. Dendy, I 

 undertook an investigation into the structure known as 

 Reissner's fibre, which, Sargent's work notwithstanding, 

 is still regarded by many as an artifact or as a coagulum 

 of cerebro-spinal fluid, Johnston even, in his recent work 

 on the "Nervous System of Vertebrates," dismissing the 

 subject with the briefest of notices. 



.As a more than ordinary interest attaches to the descrip- 

 tion of any hitherto unrecorded feature in such a well- 

 known type as the frog (i?. temporaria), I venture to 

 direct the attention of anatomists and physiologists to the 

 fact that this animal possesses a well-developed and easily 

 demonstrable Reissner's fibre, although no reference to this 

 structure can be found in Gaupp's exhaustive treatise, nor 

 does Sargent include any Anuran form in the long list 

 given by him of animals in which he has seen the fibre. 



The whole or parts of the central nervous system of 

 about a dozen frogs have been sectionised, the sections 

 having been cut in the usual three planes, and the 

 fibre lias been found in all the series examined, showing 

 very definite and constant relations to the various brairt 

 structures. In every case it may be made out, beginning 

 anteriorly at the dorsal end of the deep and narrow 

 ependymal groove on the anterior face of the posterior 

 commissure, and, emerging from this groove ventral 

 the commissure, lying freely in the iter spanning the 

 cavity between posterior commissure and cerebellum. It 

 is somewhat closely applied to the ventral surface of the 

 cerebellum, but posteriorly it slopes steeply towards the 

 floor of the fourth ventricle and continues backward, with 

 a wavy course throughout its length, to the hind end of 

 the spinal cord near the floor of the canal. 



Beneath the posterior commissure, in one specimen at 

 least, it may be observed dividing into two or more finer 

 fibres, while in the posterior half of its length numerous 

 fine fibre-like structures may be made out, apparently 

 given off to enter the substance of the spinal cord (much 

 as Sargent has described for other forms), but that these 



