;54 



NATURE 



[February io, 1898 



ray. This is of course very different from the focal distance for 

 the luminous rays. I at first^used a lens of 6 cm. electric focal 

 distance, but this did not improve matters sufficiently. I then 

 used one with a longer focus, i.e. 13 cm., and this gave satis- 

 factory results." 



The value obtained for \x. was 2 04, while the optical refractive 

 index for the D line was i "53. According to Maxwell's relation, 

 the specific inductive capacity K should therefore be 4 '16 = ju', 

 a value well within the extremes of 27 and 9*8 mentioned 

 above. It is interesting to note that the refractive power of 

 glass is higher for these electro-magnetic waves than for light, 

 and that ordinary lenses must therefore converge these waves to a 

 shorter focus. Hence the small dimensions of Bose's apparatus. 



Total Reflection of Electric Waves. 



These and some of the earlier experiments were repeated with 

 two semi-cylinders separated by an air-space, and the thickness 

 of air necessary to produce total reflection was determined. In 

 optics, a very thin film of air suffices. In the case of electro- 

 magnetic waves as produced in the laboratory, the thickness is 

 found to reckon by several millimetres. 



Two semi-cylinders of glass, with a radius of 12*5 cm., were 

 placed on the spectrometer circle. The plane faces were 

 separated by a parallel air-space. The radiator was placed at 

 the principal focus of one of the semi-cylinders ; the rays emerged 

 into the air-space as a parallel beam, and were focussed by the 

 second semi-cylinder on the receiver placed opposite the radiator. 



The two semi-cylinders were separated by an air-space 2 cm. 

 in thickness ; this thickness was found to be more than sufficient 

 for total reflection. The experiments were commenced with an 

 angle of incidence of 30° (slightly greater than the critical angle). 

 The receiver, which was placed opposite the radiator, remained 

 unaff"ected as long as the rays were totally reflected. But on 

 gradually diminishing the thickness of air-space by bringing the 



Fig. 2.— l is the lens to render the incident beam parallel ; Pi p', are the 

 right-angled isosceles prisms ; A and li are the two positions of the 

 receiver. The receiver-tube is not shown in the diagram. 



second semi-cylinder nearer the first (always maintaining the 

 plane surfaces of the semi-cylinders parallel), a critical thickness 

 was reached when a small portion of the radiation began to be 

 transmitted, the air-space just failing to produce total reflection. 

 The beginning ai transmission could easily be detected and the 

 critical thickness of air determined with tolerable accuracy. The 

 slight discrepancy in the different determinations was due to the 

 unavoidable variation of the sensitiveness of the receiver. When 

 the thickness of air was reduced to 14 mm. the receiver began 

 occasionally to be affected, though rather feebly. But when the 

 thickness was reduced to 13 mm. there was no uncertainty ; a 

 measurable, though small, portion of the radiation was now 

 found to be always transmitted. 



With an angle of incidence of 66° the minimum thickness for 

 total reflection was found to be between 7 "6 mm. and 7*2 mm. 

 The mmimum effective thickness is thus seen to undergo a 

 diminution with the increase of the angle of incidence. 



The author also determined the influence of wave-length, 

 using three different radiators. 



The following method of experimenting was adopted as offer- 

 ing some special advantages. If a cube of glass be interposed 

 between the radiator and the receiver placed opposite to each 

 other, the radiation striking one face perpendicularly would be 

 transmitted across the opposite face without deviation and cause 

 a response in the receiver. If the cube be now cut across a 

 diagonal, two right-angled isosceles prisms will be obtained. If 

 these two prisms were now separated slightly, keeping the two 

 hypothenuses parallel, the incident radiation would be divided 

 into two portions, of which one portion is transmitted, while the 

 other portion is reflected by the air film in a direction at right 



NO. 1476. VOL. 57] 



angles to that of the incident ray, the angle of incidence at the 

 air-space being always 45°. The transmitted and the reflected 

 portions would be complementary to each other. When the 

 receiver is placed opposite to the radiator, in the A position, the 

 action on the receiver will be due to the transmitted portion ;. 

 but when the receiver is placed at 90°, or in the B position, the 

 action on the receiver will be due to the reflected portion. The 

 advantage of this method is that the two observations for trans- 

 mission and reflection can be successively taken in a very short 

 time, during which the sensitiveness of the receiver is not likely 

 to undergo any great change. In practice three readings are 

 taken in succession, the first and the third being taken, say, for 

 transmission and the second for reflection. 



When the prisms are separated by a thickness of air-space 

 greater than the minimum thickness for total reflection, the rays 

 are wholly reflected, there being no response of the receiver in 

 position A, but strong action in position B. As the thickness is 

 gradually decreased below the critical thickness, the rays begin 

 to be transmitted. The transmitted portion goes on increasing 

 with the diminution of the thickness of air-space, there being a 

 corresponding diminution of the reflected component of the 

 radiation. When the thickness of the air-space is reduced to 

 about 0'3 mm. , no reflected portion can be detected even when 

 the receiver is made extremely sensitive. The reflected com- 

 ponent is thus practically reduced to zero, the radiation being 

 now entirely transmitted ; the two prisms, in spite of the breach 

 due to the air-space, are electro-optically continuous. This is 

 the case only when the two prisms are made of the same sub^ 

 stance. If the second prism be made of sulphur, or of any other 

 substance which has either a lower or a higher refractive index, 

 there is always found a reflected portion even when the two 

 prisms are in contact. 



The results obtained show that the effective thickness of the 

 air-film increases with the wave-length. This was to be expected, 

 since at very small wave-lengths, such as those of ordinary 

 light, the thickness required for total reflection becomes very 

 small. The brilliant reflection in the crack of a pane of glass 

 is a familiar example. 



PALAEOLITHIC MAN} 

 T N the address of last year the evidence for the existence of 

 -'■ man in the Tertiary period was reviewed, and although 

 some of the evidence was very cogent, yet in no case did it 

 amount to a proof, such as is necessarily demanded before so 

 great an antiquity can be accepted for the human race. On the 

 other hand, the presence of man in Quaternary times has long 

 since been proved by the presence of many undoubted flint im- 

 plements, in cave and river deposits of Pleistocene age and in 

 relation with the bones of the mammoth and other extinct 

 mammalia. 



But bther questions have now to be answered. What were 

 the physical and intellectual peculiarities of the men who made 

 the palaeolithic implements ? Have any parts of his skeleton 

 yet been found ? 



Human bones and skeletons, more or less imperfect, supposed 

 to be of Pleistocene age, have often been recorded both in this 

 country and also on the continent of Europe ; but a close in- 

 vestigation has, in most cases, proved them to be of much more 

 recent origin, or has shown that there were very grave doubts 

 as to their authenticity. 



Much has been done to eliminate the doubtful records by 

 such writers as Prof. Boyd Dawkins, M. Gabriel de Mortillet, 

 and MM. Fraipont and Lohest ; and consequently it is only 

 necessary, at the present time, to consider the more important 

 of these discoveries, and especially those which have been made 

 within the last ten or fifteen years. 



The famous Canstadt skull, described by Jaeger in 1835, is of 

 uncertain origin, for when the mammalian remains, with which 

 it was supposed to have been associated, were first described in 

 the year 1700, no mention was made of this skull, and it is 

 therefore by no means certain that it was associated with these 

 extinct mammals. A new interest is awakened in this and some 

 other of the earlier and unauthenticated remains of man by the 

 discovery, within the last twelve years, of very similar skulls 

 which are accepted as of paleolithic age. The skull discovered 

 by M. Faudel in 1865, at Eguisheim on the Lower Rhine, is not 

 unlike that from Canstadt, and is generally believed to be of 

 1 Abstract of Presidential Address to the Geologists' Association, delivered 

 at the Annual Meeting, February 4, by Mr. E. T. Newton, F.R.S. 



