May 27, 1897] 



NA rURE 



79 



LETTERS TO THE EDITOR. 



[ The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of rejected 

 manuscripts intended for this or any other part of Nature. 

 IVo notice is taken of anonymous communications.] 



Adjustable X-Ray Tubes. 



The writer has discovered a method of adjustment for X-ray 

 tubes, somewhat difierent to those he has previously described, 

 which possesses distinct practical advantages. 



The arrangement is as shown in the illustration, and its 

 essential feature consists in mounting the kathode upon a 

 sliding support, so that it can be moved axially to a very small 

 extent in and out of a tubular neck, blown on one end of a glass 

 bulb. VVhen arranged in this manner, the exact position of the 

 kathode is found to have an enormous influence upon the pene- 

 trative value of the X-rays produced. With a suitable and 

 constant degree of e.xhaustion, if the kathode is placed as shown 

 in full lines in Fig. i, X-rays of very high penetrative value 

 are produced, while the small movement of about -3 inch 

 required to place it in the position indicated by the dotted lines, 

 will suffice to reduce the penetrative value of the rays almost to 

 nothing. Between these two extremes every grade of penetra- 

 tive value is readily obtained by simply altering the position of 

 the kathode between its limits of travel. If the tube be used in 

 a horizontal position, this can easily be done by merely tapping 

 it at one end or the other, without removing it from its support, 

 and the anti-kathode being a fixture, the point of origin of the 

 X-rays remains always in the same position. In this manner, 

 with a single tube, and without alteration to the vacuum, X-rays 

 of any desired penetrative value can readily be obtained, while 



Boomerangs without Twist. 



In your issue of May 13 (p. 46), a writer mentions some 

 Australasian boomerangs as thrown not to return (if I have 

 rightly understood him). I had not heard of these before, but 

 in British India at least one race, the Kolis of Northern Gujarat, 

 have the like. These are invariably of " fish " section, varying 

 in vveight, curve, and material ; but the commonest and most 

 efficient sort is of " Babul " wood (Acacia arabica), with the 

 natural curve of the heart of the wood, something like that of 

 an old-fashioned genuine Turkish sabre, rather a *' knee" than 

 any regular geometric curve. 



They are used with great effect on ground-game ; much less, 

 of course, on birds. In one case an old and feeble man, 

 threatened by a swordsman, cut the assailant's shins across with 

 his boomerang at about ten paces, and brought him down. 

 Before the astonished thief could rise, the now unarmed old 

 man had disarmed and literally taken him captive. 



I tried the case myself, gave the swordsman two years, and 

 the boomerang man much honour. Certain people called 

 Maravars, in the south of the Madras Presidency, are said to 

 have similar boomerangs ; but I have not seen them. 



Many Indian races throw straight sticks at game habitually, 

 and even in a city riot the thrown sticks are often more dan- 

 gerous than those in hand (bar iron-bound clubs). But their 

 practice is not scientific, like boomerang-throwing. 



W. F. Sinclair. 



102 Cheyne Walk, Chelsea, S.W., May 14. 



Scorpion carrying Flower. 



One evening last February, while sitting in the verandah of 

 my house at Aden, my attention was drawn to an object advanc- 

 ing across the floor, which seemed to be some peculiar leaf 



at the same time it is possible to compensate for" the uncontrol- 

 lable variations in vacuum that are found to occur in practice, 

 and thus to maintain the penetrative value of the rays at any ; 

 desired degree. 



The effect is evidently due to changes in the electrical resist- 

 ance of the tube, which, as measured by the alternative spark in ! 

 air, is much highest when the kathode is in the position shown 

 in full lines, i.e. that which gives rays of the greatest penetra- 

 tion, and appears to be closely connected with the proximity of 

 the kathode to the glass, which is greatest in the position just 1 

 mentioned. | 



This factor is evidently so important that it much more than S 

 neutralises the effect of the alteration to the distance between 

 kathode and anti-kathode, which, as the writer has previously 

 shown, has the result of increasing the penetrative value of the | 

 rays, and also the resistance of the tube, the nearer these two [ 

 electrodes are together. 1 



In order to obviate the overheating of the anti-kathode, which 

 is a considerable source of trouble in all X-ray tubes when much | 

 electrical power is employed, the writer finds it con%'enient to 

 mount the ordinary small disc of platinum upon a considerably j 

 larger and thicker disc of aluminium, say about i inch in diameter ! 

 and -25 inch in thickness, the platinum being tightly wedged | 

 and riveted in a shallow circular depression turned out of one 

 face of the aluminium disc. Owing to the considerable radiat- 

 ing surface of the aluminium, and its considerable mass, this 

 arrangement prevents the platinum from attaining an excessive 

 temperature. In addition, the platinum having only one surface 

 exposed, the tubes do not blacken so quickly. 



May 17. A. A. C. SwiNTON. 



NO. 1439, VOL. 56] 



insect or phasma. On looking at it closer I saw it to be a 

 scorpion (identified by Mr. Pocock from my description as 

 Parabuthus liosoma), which was holding over its back by one 

 claw a large blossom of Poinciana regia, known in Aden as the 

 white-gold mohur tree. Its tail, curled over its back, further 

 assisted in retaining the flower in position. The nearest tree 

 from which it could have obtained it was at least 30 feet away, 

 and to bring it the scorpion must have carried it over alow stone 

 parapet and up two or three steps, so that intention seems to 

 be proved. What that intention was it is hard to define. 

 Hardly for concealment, for the size of the flower made it more 

 conspicuous ; besides, it was night. If it was the lamp-light it 

 wanted to avoid, it is necessary to assume that, finding the light 

 too strong, it went back to get the flower. It could hardly be 

 as food, for scorpions are not known to live on vegetable sub- 

 stances ; nor, as far as I know, do they construct nests. I regret 

 that I did not allow the creature to reach its destination, and so 

 ascertain its intention ; but, unfortunately, I gave in to my first 

 impulse and crushed it. My wife suggested that perhaps it was 

 going to a wedding, but this explanation is more poetical than 

 scientific. 



Some of your readers may be able to throw a little light on 

 this curious instance ; but Mr. Pocock, of the British Museum, 

 to whom I related the above, said he had never heard of a similar 

 case. A. Newnham. 



The Utility of Specific Characters. 



Mr. Cockerell asks whether it is possible to explain right- 

 handedness, the dextral or sinistral coil of snail-shells, and similar 

 features as having any utility to the species, and he then speaks of 



