462 



NATURE ^\ 



[March 20, 1890 



Instruments in actual use for what have now become 

 every-day purposes are fully illustrated and described. 



The book is well up to date both in the experimental 

 and applied branches. Mr. Shelford Bidwell's apparatus 

 for studying the changes in length of a bar during mag- 

 netization is described in such a way as to make the 

 object of the experiment and the method of carrying it 

 out easily understood. More of this kind of thing in our 

 text-books is very desirable as showing that progress in 

 a science is not made by chance, but is the outcome of 

 careful thought on the part of patient investigators. 



As a text-book for classes where experimental work is 

 encouraged it is especially suitable, but we recommend 

 it to the notice of all beginners. Numerous questions and 

 specimen answers follow the various chapters, and an 

 appendix gives instructions for making simple apparatus. 



Astro7totny with an Opera-Glass. By Garrett P. 

 Serviss. (London and New York: D. Appleton and 

 Co., 1889.) 



We are glad to welcome this, the second edition of a 

 popular introduction to the study of the heavens. The 

 author has surveyed, with the simplest of optical instru- 

 ments, all the constellations visible in the latitude of 

 New York, and carefully noted everything that seemed 

 of interest to amateur star-gazers. In addition to the 

 map and directions given to facilitate the recognition of 

 the constellations and the principal stars visible to the 

 naked eye, many facts are stated concerning the objects 

 described which render the work a compendium of useful 

 and interesting information — an astronomical text-book 

 as well as a star-atlas. Similar combinations are very 

 desirable introductions to every science, and offer the 

 best means of extending true knowledge. To lead the 

 student to Nature, and direct his attention to some of 

 her marvellous works, to make him see natural pheno- 

 mena intellectually, should be the basis of all scientific 

 instruction, and works constructed on these lines read 

 like story-books. With such works the one before us 

 should be included, and there could hardly be a more 

 pleasant road to astronomical knowledge than it affords ; 

 replete with information, elegant in design, easy of 

 reading, and practical throughout, it deserves to rank 

 high among similar guides to celestial phenomena. A 

 child may understand the text, which reads more like 

 a collection of anecdotes than anything else, but this 

 does not mar its scientific value, and if the work multi- 

 plies the number of observers, as it is calculated to 

 do, the dearest wish of every astronomer will be gratified. 



LETTERS TO THE EDITOR. 



[ The Editor does not hold himself responsible for opinions e^ - 

 pressed by his correspondents. Neither can he undertaii 

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

 manuscripts intended for this or any other part of Natukk, 

 No notice is taken of anonymous communications. ] 



Electrical Radiation from Conducting Spheres, an 

 Electric Eye, and a Suggestion regarding Vision. 



I DO not know how far the description of little isolated ex- 

 periments is serviceable, but I am tempted to communicate a 

 simple plan I use for exciting electric oscillations in dumb-bells, 

 ellipsoids, elliptical plates, .spheres, or other conducting bodies 

 of definite geometrical shape unhampered by a bisecting spark- 

 gap. I do it by supplying electricity to opposite ends of the 

 conductor by means of Leyden jar knobs brought near enough 

 to spark to it : said knobs being likewise connected with the 

 terminals of a small Ruhmkorff coil. The charge thus supplied 

 or withdrawn at every spark settles down in the conductor 

 after a few oscillations, and these excite radiation in surrounding 

 space. 



There are many ways of arranging the Leyden jars : some 

 more effective than others. The outer coats of the two jars may 



or may not be connected together. Connecting them in some 

 cases brightens the sparks at short range, but seems to have a 

 tendency to weaken them at long ranges. It is not difficult to 

 surmise why this is so. 



Of course, when the outer coats are disconnected, only an 

 insignificant portion of the capacity of the jars is utilized ; but 

 unless the thing to be charged has too large a capacity it works 

 perfectly well. 



The receiver or detector is a precisely similar conductor 

 touched to earth by a point held in the hand. The distance at 

 which such a receiver responds is surprising. Or one may use a 

 pair of similar conductors and let them spark into each other; 

 but this plan is hardly so sensitive, and is more trouble. 



The fact of being able in actual practice to get radiation from 

 a sphere, is interesting, inasmuch as the subject of electrical 

 oscillations in a perfectly conducting sphere has been worked out 

 accurately by Prof. J. J. Thomson in the London Mathematical 

 Society's Proceedings. I have not the volume by me, but I 

 think he reckons the period of oscillation as the time required by 

 light to travel i '41 diameters of the sphere. 



The case of spheres of ordinary metal will not be essentially 

 diflferent, with these rapid oscillations, for the electric currents 

 keep to a mere shell of surface in either case ; and in so far as 

 damping affects the period, the dissipation of energy by radiation 

 (which is common to both) is far greater than that caused by 

 generation of heat in the skin of a metal sphere. 



I happen to have four similar spheres of nickel-plated metal 

 on tall insulating stems; each sphere 12 "i centimetres in dia- 

 meter. Applying spark knobs to each end of a diameter of one 

 of them, and applying the point of a penknife to another one 

 standing on the same table at a distance of two and a half 

 metres, I am able to get little sparks from it without using any 

 reflector or intensifier. 



Or arranging three spheres in a row, with intervals between 

 and knobs outside, 5 short, spaikgaps in all (see figure), andi 



using a fourth sphere as detector of this triple-sourced radiation^ 

 I draw little sparks from it to a touching penknife at a distance 

 of 12 feet (366 centimetres, actual measurement). 



In this case it may be a trifle better to hold one's hand near 

 the receiving sphere at the side opposite to the penknife, and 

 thus vary its capacity by trial so as to imitate the disturbing 

 effect of the contiguous spheres in the transmitter. 



The complete waves thus experimented on and detected are 

 only 17 centimetres (six and a half inches) long, and I imagine 

 are about the shortest yet dealt with. 



But we do not seem near the limit set by lack of absolute 

 suddenness in sparks yet, and are going on to try a large 

 number of little globes. 



Exciting a lot of little spheres by a coil in this way forcibly 

 recalls to mind the excitation of a phosphorescent substance by 

 a coil discharge. 



And a receiver not very unlike the rod- and- cone structure of 

 the retina can likewise be made. My assistant has been experi- 

 menting on a sort of gradated receiver which he made himself. 

 I have recently had made a series of long cylinders with 

 diameters ranging above and below 12 centims. ; and the 



