340 



NATURE 



[February 13, 1896 



all calculations of dimensions depend, and therefore it 

 will be valuable as an introduction to works concerned 

 with special practical applications of the rules described 

 and exemplified. Moreover, it should play a useful part 

 in schools, by illustrating the concrete applications of 

 abstract geometrical principles. The large number of 

 original examples will be found of great assistance by 

 teachers, and the questions, selected from papers set by 

 the principal examining bodies, will prove of service as 

 tests of the students' capabilities in working out men- 

 suration problems. 



Of a less detailed character is the Rev. Dawson 

 Clarke's primer, intended " for the use of schools, and 

 Woolwich, Sandhurst, and Home Civil Service candi- 

 dates." The book is a collection of rules and formula;, 

 with examples to explain their use, and numerous 

 exercises selected from various examination papers. It 

 particularly appeals to students who learn the rules of 

 mensuration in order to utilise their knowledge in the 

 examination-rooms of the Civil Service Commissioners ; 

 but it is, also, a concise text-book which other students 

 will find serviceable. 



Physical Measurements. By Frank C. Weedon. Pp. 



232. (London : G. Gill and Sons, 1895.) 

 This volume is another help towards the establishment 

 of rational methods of instruction in elementary science. 

 It is a laboratory manual of practical physics for organised 

 science schools under the Department of Science and 

 Art, and other secondary schools. Of the educational 

 value of the course contained in the book, there can be 

 no doubt ; for the experiments (which are of a character 

 suited to beginners) follow a natural order, and are such as 

 will develop the faculties of observation, investigation, and 

 common sense ; in fact, they will lead the student to think 

 as well as learn. The book is divided into three sections, 

 dealing respectively with measuring and weighing, rela- 

 tive densities, and experimental mechanics. Experi- 

 ments on these matters elucidate the fundamental prin- 

 ciples which fonn the basis of a scientific education. 

 The knowledge cannot be labelled " Sound, Light and 

 Heat," or " Magnetism and Electricity," and therefore 

 superficial critics, and syllabus-bound teachers, think it 

 is not Physics. We are of the opinion, however, that 

 experimental work in measuring and weighing, constitutes 

 the foundations of physics. The student who is able to 

 weigh and measure carefully, and to observe and think 

 accurately, knows more of the realities of physical 

 investigation than if he had spent a dozen years in learn- 

 ing scraps of information about other people's contribu- 

 tions to knowledge. 



LETTERS TO THE EDITOR. 



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

 pressed by his correspondents. Neither can he tindertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications .'\ 



The New Actinic Rays. 



A BRIEF account of some experiments which I have been 

 making in my laboratory at Blythswood, in connection with the 

 new photographic rays, may, I hope, be of interest to the readers 

 of Nature. 



Three or four years ago I constructed a very powerful 

 Wimshurst electrical machine. It has 128 plates, three feet in 

 diameter, and is driven by an electric motor of about i^ horse- 

 power. With this machine, which was specially built for 

 quantity, I can obtain a torrent of sparks a foot and a half or 

 two feet long : and it occurred to me to try to obtain photo- 

 graphs, after the manner of Rontgen, but without the interven- 

 tion of a vacuum tube. 



A thick sheet of lead was placed upright between the poles of 

 the electric machine, as a screen, and was connected to the 



NO. 1372, VOL. 53] 



ground, the two poles being insulated. A sensitive dry plate 

 was put into the camera dark slide, with a metallic object to be 

 photographed (a steel washer with holes in it), and this was 

 connected, by a wire which passed out of the dark slide, to the 

 ground. The whole was wrapped up in four folds of a black 

 velvet focussing cloth, and was put, in some cases between the 

 negative pole and the lead screen, and in other cases between 

 the positive pole and the lead screen, the plane of the slide being 

 perpendicular to the line of discharge. In all cases good strong 

 negatives were obtained with exposures of about twenty minutes. 

 The machine was arranged to give a silent brush discharge 

 during the experiments. 



I next tried similar experiments with the dark slide containing 

 the sensitive plate quite out of the line of discharge, and with 

 the plane of the plate parallel to the line of discharge, and 

 obtained equally good results. It seems, therefore, that the 

 vacuum tube is not essential to the production of the Rontgen 

 rays. With reference to this, however, I am not so sure, as I 

 think I may have been deceived by using isochromatic plates — 

 at all events I am engaged in further experiments either to 

 confirm or the contrary. Blythswood. 



Blythswood, Renfrew, February 10. 



With reference to Mr. Porter's letter regarding the amount 

 of electric energy and exposure required for obtaining photo- 

 graphs by the Rontgen method, I may mention that against his 

 photograph taken with a 3-inch coil and four minutes' exposure, 

 I can instance a successful human foot that shows the bones very 

 distinctly almost up to the ankle-joint, in taking which I used a 

 lo-inch coil working at about half power without Leyden jars, 

 and for which fifty-five seconds' exposure proved ample. 



For living physiological subjects, it is very important to 

 shorten the exposure as much as possible, and to attain a mini- 

 mum in this respect, very high vacua and considerable E. M. F. 

 are requisite. 



Again, for an extensive subject, a large tube placed at a con- 

 siderable distance from the subject is required, and more electric 

 energy is needed for this than for a small subject, for which a 

 smaller tube in closer proximity will suffice. 



66 Victoria Street, S.W. A. A. C. Swinton. 



Havinc; made some experiments on the lines laid down by 

 Mr. Gilford, of Chard, I think the two enclosed photographs 

 will prove of interest, as showing perhaps that Mr. Gifford's 

 method of dispensing with a Crookes' tube introduces elements 

 of another character. Both these negatives were taken without 

 a tube, using the discharge from the terminal of a small Tesla 

 transformer. In each case a metal plate was placed behind the 

 film in communication with the other terminal of the coil. 

 Under these conditions a stream of "discharge" passes from 

 one terminal through the photographic film. 



The interesting point is that not only does the outline of the 

 coin come out, but also the impression. And that in the case of 

 the florin the coin was placed behind the film. The same spark- 

 ing appearance as described by Mr. Gilford is evident. 



From the fact that it is immaterial on which side of the photo- 

 graphic film the coin is placed, it is evident, I think, that we 

 have here to do with a " contact " phenomenon, and not with 

 Rontgen's rays at all. Sydney D. Rowland. 



38 Wimpole Street, W., February 2. 



"The Astronomical Theory of the Glacial Period." 



As it was my two letters which initiated the interesting and 

 not unfruitfiil discussion now going on in your pages on the 

 above subject, I think it right to say a few words in reply. 



The object of my letters was to point out (perhaps I did it in 

 somewhat too heated language) that Sir Robert Ball, whose 

 personal and official distinction give his words exceptional 

 weight, had in his work entitled " The Cause of an Ice Age " 

 given fresh currency to a discredited theory, and further that 

 when this had been pointed out, he had refused to take any 

 notice of his critics, and continued to publish his book. 



In his letter to you, Sir R. Ball (if I do not misunderstand 

 him) entirely breaks away from the position maintained in his 

 book, and gives up the case there argued, definitely and com- 

 pletely. While Prof Darwin, who had given the book the 

 advantage of his friendly recommendation and countenance, tells 

 us he is now reluctantly compelled to take the other side. 



