186 



KNOWLEDGE. 



May, 1913. 



possible to obtain interference effects with them, by using a 

 crystal as a diffraction grating. As the atoms of a crystal are 

 arranged regularly, and the spaces between them have about 

 the same relation to the supposed wave length of the rays as 

 the " constant " of a diffraction grating has to that of visible 

 light, a crystal would thus form a most perfectly ruled grating 

 for them. In order to test this practically the aid of photo- 

 graphy was called in. In the first place, however, it was 

 necessary to obtain as far as possible a parallel beam of 

 X-rays, so that the conditions should be the same as 

 when a parallel beam of light is allowed to fall upon 

 the grating of a spectroscope. This was accomplished 

 'as well as could be," by means of holes pierced in sheets of 

 lead, the X-rays being finally transmitted by an aperture 

 of one millimetre in diameter, and thus a very narrow pencil 

 was obtained. This was allowed to fall upon the crystal, and 

 behind it, at a distance of about three centimetres, a photo- 

 graphic dry plate was placed perpendicular to the beam of 

 rays. The rays, after traversing the crystal, fell upon the 

 plate, and this, after the necessary exposure (which amounted 

 in some instances to several hours), was developed. The 

 effect produced was shown as a dark, circular spot, surrounded 

 by a series of much weaker ones, arranged in regular order. 

 By altering the distance of the photographic plate from the 

 crystal, the spots could be made to approach, or recede from, 

 the central one, so that it appeared clear that they were due 

 to narrow rectilinear pencils spreading outwards from the 

 crystal. It was found that the angles some of these made 

 with the undeviated rays were as much as forty degrees. It 

 was also noticed that the spots scarcely altered in size as the 

 distance between the plate and the crystal was increased, but 

 remained of the size of the smallest aperture employed for the 

 transmission of the rays. And whatever the final result of 

 the experiments may lead to, whether they solve the problem 

 of crystal structure as well as that of the true nature of 

 X-rays, there is no doubt but that photography has again 

 rendered great service in showing that which otherwise 

 might have remained of theoretical interest alone. 



FIXED FOCUS CAMERAS.— Fixed focus is the term 

 applied to hand cameras in which the plate is placed at a 

 fixed distance from the lens, so that no focusing of the image 

 can be accomplished. In many cases the plate is at a distance 

 greater than the focal length of the lens ; in other cases it is 

 at the focal length itself. Now, in order for an image to 

 appear sharp, it is generally considered that any point in the 

 image of an object must not exceed one hundredth of an inch 

 in diameter. Therefore, the distance of the object must be 

 such that these points, " termed discs of confusion," do not 

 exceed this value in the image. When the plate is at a 

 distance from the lens greater than its focus, then a slight 

 amount of confusion exists for objects at infinity. If, however, 

 the plate is placed at a distance from the lens equal to its 

 focal length, then all objects from a certain near point to 

 infinity are in sharp focus. In order to find this point the 



following formula is employed : — — — where / = the 



a 



the focal length of the lens, and a the value of the stop. Thus, 



suppose with a lens of five inches focus and stop F/6 it is 



desired to find the nearest point beyond which everything else 



would be in focus, then = 34-7 feet. 



12 X 6 



Therefore, all objects from this distance to infinity would be 



sharply focused : that is, the circles or discs of confusion 



forming the points in the image of the objects would not be 



greater than one hundredth of an inch in diameter with the 



stop employed. When the object is nearer than this, then the 



image will be at a greater distance, and will agree with 



the case first stated, and although there may be certain 



advantages attending it, the method given for calculating that 



of the object would no longer be correct, as the formula only 



holds good when the plate occupies the position of the 



principal focal plane of the lens. There are, doubtless, many 



cases in which strict attention to theoretical details need not 



be paid, this, of course, depending largely upon individual 



ideas ; but it should always be remembered that when images 



are intended for enlargement either by means of lantern 

 slides or otherwise, the results are likely to suffer in a serious 

 manner if too little attention be given to obtain a sufficiently 

 sharp image in the first place. 



PHYSICS. 



By Alfred C. G. Egerton, B.Sc. 



RONTGEN RADIATION.— When the cathode rays— the 

 rays emitted in a vacuum tube normally to the cathode — were 

 originally discovered, some considered them to consist of 

 small discrete particles, and others considered them to be 

 waves in the ether. Professor Sir J. Thomson's researches 

 eventually proved without doubt that the rays consisted of 

 negative charges emitted with great velocity carried by 

 corpuscles of mass, one seventeen-hundredth of the mass 

 of the atom of hydrogen. The cathode ra5's, when they 

 impinge on a substance, are stopped, provided the thickness 

 of the substance is sufficient, and they give rise to another 

 penetrating type of radiation called the Rontgen rays, after 

 their discoverer. The rays evolved are rays of mixed 

 penetrating power, but for every metal there is also a 

 definite radiation, characteristic of that metal and of definite 

 penetrating power; the higher the atomic weight of the 

 element, the greater the penetrating power of the charac- 

 teristic radiation. Now, the Rontgen or X-rays give rise to 

 secondary rays on being allowed to impinge on a metal, and 

 these rays, too, are found to be characteristic of the metal, 

 their velocity varying with the atomic weight of the metal. 

 Thus, a cathode ray gives rise to an X-ray, and an X-ray to a 

 secondary ray of the same type as the cathode rays. The 

 7 -radiation from the radio-active substances is also in all its 

 properties similar to a very penetrating Rontgen radiation. 



Now, discrete electrically-charged particles, be they positive 

 or negative, are affected and bent out of their path by magnetic 

 or electric fields, but the X-rays or T-rays are not affected by 

 such fields. Consequently, the rays must be either neutral 

 and consist of a pair of equal and opposite electrical charges, 

 or consist of a pulse (a kind of single wave) in the ether. It 

 cannot be said that complete proof has been adduced in favour 

 of either view. But the latter theory explains most easily the 

 majority of the facts, while the former has for support the 

 ready convertibility of the X-rays and " secondary " p radia- 

 tion, though with the pulse theory there is no great difficulty in 

 considering that since an electron possesses around it an 

 electro-magnetic field, and when stopped it is the inertia 

 of this field which continues in motion and gives rise to a 

 pulse in the ether. The energy of the pulse may be absorbed 

 by obstructing matter, but addition of energy to an atom in 

 that way is almost similar to adding an electron, and so 

 an electron is released from the atom and forms a secondary 

 ray. Recently, Professor Barkla has been able to demonstrate 

 the reflection of X-rays from the cleavage planes of a crystal 

 of rock salt. The narrow pencil of X-rays was allowed to fall 

 at grazing incidence, and the principal secondary pencil of 

 rays formed was one obeying the laws of reflection from the 

 cleavage planes. It has also been possible to demonstrate the 

 formation of Interference Fringe systems : a diverging beam 

 of radiation was so directed on to a crystal that various 

 portions of the beam were directed at different angles on the 

 crystal cleavage planes ; it was found that the intensity of the 

 reflected pencil varied periodically with varying angle of 

 incidence, and that such maxima of intensity that were 

 obtained could be explained by the interference of the various 

 reflected radiations from different equal spaced parallel planes 

 within the crystal. The wavelength works out to be 

 •6X 10 ~' J centimetres which agrees with the probable wave- 

 length of the X -radiations from other considerations. 



These experiments have added to many others carried out 

 by Professor Barkla and his collabrators in favour of the 

 Ether pulse theory of the Rontgen rays. 



"THE DYNAMICS OF PIANOFORTE TOUCH."— 

 This is the title of a paper by Professor Bryan read before 

 the Physical Society on February 14th ; the automatic piano 

 player, unlike the gramophone, has not been given great 



