370 



KNOWLEDGE. 



September, 1910. 



violet, and this he does with an aesculine screen. The 

 practical details are too long for reproduction here, but they 

 will, doubtless, soon be generally available. If plates so 

 treated prove as ad\'antageous as the author has found them, 

 there seems to be no reason why autochrome, and perhaps 

 other colour plates, should not be commercially supplied with 

 suitable compensating screens. A reduction of the time of 

 e.xposures to one-eighth would be an enormous advantage in 

 almost all the work for which such plates are employed. 



THE RESOLVING POWER OF PLATES.— In February 

 last, I referred to a paper by Dr. Mees on this subject. Since 

 then Dr. W. Schetfer has published (fin7. Jul. Phot.. lylO. 

 453 and 576) the results of his experiments ha%ing the same 

 object in view, but carried out in a rather different manner. 

 Some of his conclusions seem so obviously true that many 

 have probably taken them for granted and based their 

 procedure upon them. But this in no way detracts from the 

 value of an experimental proof of their truth. Dr. Scheffer 

 employed as the object cross-lined screens, the opaque lines 

 and the transparent spaces being of equal width, and also 

 similarly ruled screens but with parallel lines only. A small 

 image of this produced by a lens that was proved to have a 

 greater resolving power than necessary, was photographed in 

 a specially made camera, with due precautions as to focussing, 

 and photo-micrographs were made of the resulting images. 

 The communications are copiously illustrated with reproduc- 

 tions. 



Each grain of silver salt upon which the light image falls, 

 becomes luminous, and shines upon the adjacent particles. 

 Dr. Scheffer finds that for the same quantity of silver salt per 

 unit area, the resolving power is greater the more transparent, 

 that is, the less turbid the film is. He finds that when the 

 finest resolution is produced the image lies only in the surface 

 of the film, so that the resolution of specially fine structure 

 may be obliterated by the use of Farmer's reducer, dissolving 

 it away, without much affecting the total density of the image. 

 The lateral spreading of the light, which tends to obliterate 

 detail or reduce resolving power, depends upon (1) the light 

 scattering power of the film ; (2) the transparency of the film; 

 (3) the duration of the exposure; (4) the development. The 

 best exposure is the shortest that will give a good image on 

 development, but ten times this exposure deteriorates the 

 resolution to a very small extent. One hundred times the 

 exposure obliterates the fine detail. The best amount of 

 development is that which would generally be considered as 

 normal. If the gelatine is coloured of a tint that absorbs the 

 active light, as yellow for ordinary plates, the lateral spreading 

 is reduced and resolving power increased, and this is a matter 

 that demands special attention in colour-sensitised plates, in 

 which the film itself often remains coloured. 



RADIO-ACTIVITY OF POTASSIUM COMPOUNDS.— 

 The radio-activity of potassium compounds appears to be well 

 established, and the failures to detect it due to the use of 

 insufficiently sensitive methods. J. Elster and H. Geitel 

 (Physikal Zcitsch, 1910, 275), ha\e tried various methods 

 to concentrate the activity of potassium salts, acting on the 

 assumption that the activity might be due to some other 

 substance than potassium (an impurity) present in all the 

 tested sources of this metal. But they have failed every time 

 to get evidence of such concentration or separation, and they 

 therefore conclude that the activity is probably inherent in the 

 potassium. The activity of potassium and other common 

 metals is exceedingly slight, and doubtless would never have 

 been observed at all if direct experiment had not been led up 

 to by the properties of the notably radio-active substances. 



PHYSICS. 



By W. D. Eggar, M.A. 



THE PRESSURE OF LIGHT.— Suppose that a stream 

 of minute corpuscles is flowing steadily against a surface 

 which absorbs them and destroys their momentum. The 

 momentum destroyed per second by unit area of the surface 

 is a measure of the pressure of the stream. Now if in is the 



mass of the corpuscles in one cubic centimetre of the stream, 

 and V the velocity of the stream at right angles to the surface, 

 it is obvious that the mass impinging on one square 

 centimetre in one second is mv, and, therefore, the 

 momentum destroyed per second by one square centimetre 

 is inv X r = div'. In other words, the pressure of light, 

 according to the corpuscular theory, should be equal to twice 

 the kinetic energy of translation per unit volume of the beam 

 of light. Many attempts were made during the eighteenth 

 century to detect the pressure of light on a delicately- 

 suspended disc ; the results were quite inconclusive. Con- 

 vection currents in the surrounding air, or, when a "vacuum" 

 was employed, the radiometer effects, subsequently discovered 

 by Sir William Crookes, interfered with the early experiments, 

 as with the later ones of Lebeden, Nichols and Poynting. 

 But the earlier experimenters, with no " principle of conser- 

 vation " for measuring the energy of their beam of light, were 

 unaware that the pressure for which they were looking was 

 far too minute for detection by the apparatus which they 

 employed. The corpuscular theory was abandoned and 

 replaced by the wave-theory ; but one of the reasons 

 sometimes given for its abandonment, the non-existence 

 of pressure, has in its turn to be abandoned. According to 

 Clerk Maxwell's electro-magnetic theory, waves of light 

 consist of electric and magnetic tubes of force at right angles 

 to the direction of the beam of light, and these tubes should 

 press on any surface on which they impinge with a pressure 

 equal to the energy in unit volume of the beam of light. In 

 1899 Professor Lebeden detected and measured the pressure 

 by letting the concentrated rays of an electric lamp fall on a 

 blackened platinum disc suspended in a vacuum sufficiently 

 high to get rid of convection and considerably to reduce the 

 radiometer action ; and, by using discs of different thickness 

 with radiometeraction proportioned to the thickness, he was able 

 to calculate the pressure on a disc infinitely thin. His experi- 

 ments, as well as those of Nichols and Hull, confirm Maxwell's 

 theory. In the Bakerian lecture delivered at the Royal Society, 

 last March, on " The Recoil from Light," Professor Poynting 

 and Dr. Barlow describe their method of measuring the 

 pressure of the beam of light against its source. If the beam of 

 light carries momentum with it, as is now proved experimentally, 

 it does not appear possible, whatever theory of light we adopt, to 

 avoid the conclusion that this momentum was derived from the 

 source of light. The experiments described seem to prove 

 this conclusion, and the authors of the lecture proceed to 

 consider the size of particles on which the pressure of light 

 would balance gravitation. As the mass of a sphere varies as 

 the cube of its radius, and the pressure of sunlight on its 

 surface as the square of the radius, it is plain that spheres of 

 radius smaller than the critical value (16 X lO"*' cm.) would be 

 pushed away. This helps to explain the phenomenon of the 

 tail of a comet. Again, in considering bodies for which 

 gravitation is greater than the light pressure, a small sphere, 

 absorbing the sun's heat and radiating its own, and at the same 

 time moving roimd the sun, will experience a retarding force, 

 due to the crowding up of the waves in front and opening out 

 of those behind. Its energy will be gradually abstracted, and 

 it tends to fall into the sun. A sphere Tn.iiT cm. in diameter at 

 the distance of the earth from the sun would fall into the sun 

 in 45,000 years. 



ZOOLOGY. 



By Professor J. Arthur Thomson, M.A. 



THE ZOOLOGICAL CONGRESS.— The Triennial Meet- 

 ing of the International Zoological Congress was held this 

 year (August 15-20th) in Graz, a Mecca that rewards pilgrims 

 not only in itself, " the pearl of the Steiermark," but in the 

 journey towards it from any direction. A well-organised series 

 of meetings, a large attendance of zoologists from all parts of 

 the Palaearctic region, an embarrassment of riches in the way 

 of papers, a wholesome as well as beautiful town, a friendly 

 and hospitable people, and plenty of sunshine, everything 

 contributed to making the eighth Congress a success. In days 

 of specialisms these meetings are more than luxuries ; they 

 enable one to keep within hearing at least of what is being 



