July 26, 1888] 



NA TURE 



307 



have been reduced. The form of reduction and publication will 

 be similar to the catalogue of bright stars, except that it will be 

 convenient to retain the " Durchmusterung " numbers and 

 places, arranging the stars in the order of the zones in that 

 catalogue. It is. hoped that the photographs for this investiga- 

 tion will be nearly all taken by the autumn of 1888, and the 

 remainder dining the following year. To provide for a possible 

 increase in sensitiveness of the plates, precedence is given to 

 those completely covering the sky once, the alternate plates, 

 covering the sky the second time, being taken later. The 

 actual improvement in the plates shows itself by an increase in 

 the number of spectra in this second series of plates. In some 

 cases over three hundred stellar spectra appear on a single 

 plate. 



3. Detailed Study of the Spectra of the Brighter Stars. — These 

 spectra are obtained by placing four prisms, having an angle of 

 about 1 5 , and each nearly a foot square, over the object-glass 

 of the 11-inch telescope, as described in the last Report. The 

 increased sensitiveness of the plates has greatly increased the 

 number of stars bright enough to produce a satisfactory image in 

 this way. The white stars of the first type give good images 

 when no brighter than the fourth magnitude. These spectra are 

 about 4 inches in length. An improvement has been made in 

 the method of enlargement with a cylindrical lens described in 

 the last Report. When such a lens was used with an enlarging 

 lens having a small aperture, the width of the spectrum was 

 greatly reduced ; with a large aperture, the best definition could 

 not be attained. A slit perpendicular to the axis of the cylin- 

 drical lens is accordingly placed over it. This reduces the aper- 

 ture in one direction so that the definition of the lines is good, 

 without affecting the width of the spectrum. Slow plates are 

 also used in the enlargements to increase the contrast. Much 

 more brilliant spectra are thus obtained. 



4. Faint Stellar Spectra. — As stated above, the 28-inch 

 reflector constructed by Dr. Draper is now ready for use. The 

 difficulties commonly encountered in the use of a large reflector 

 have been met, and it is hoped successfully overcome. A 

 spectroscope has been devised for this instrument which will 

 give a dispersion about equal to that employed in the first and 

 second of the researches described above. As the area of the 

 aperture of this telescope is about eleven times that of the 8-inch 

 telescope, it is hoped that much fainter stars can be photo- 

 graphed with it. A study will be made of the spectra of the 

 variable stars of long period, of the banded stars, and of other 

 objects having peculiar spectra. 



But little progress has been made with the other investiga- 

 tions proposed, including the reduction to wave-lengths, and 

 the study of the approach and recession of the stars. It seemed 

 best to concentrate our work on the researches described above, 

 undertaking the other investigations as soon as time permitted. 



The investigations described above are illustrated by a plate. 

 A special study was made of the spectrum nf the variable star 

 £ Persei. A variation in this spectrum would have an important 

 bearing on the theory that the diminution in light is due to an 

 interposed dark satellite. Spectra of this star at minimum were 

 first obtained with one prism. With the increased sensitiveness 

 of the plates more prisms were tried, until finally good spectra 

 were obtained with all four prisms even when the star was at its 

 minimum. At first it was thought that a variation was detected 

 in the spectrum, but this change was not confirmed under more 

 favourable circumstances. The spectrum of this star on February 

 6, 1888, when at its full brightness, is contrasted in the plate 

 with the spectrum on February 9, 1888, when the star was at 

 its minimum. A careful inspection of the original negatives 

 failed to show any differences in the spectra. Twenty lines are 

 visible at minimum, all of which are seen at maximum. The 

 spectrum of a Ononis is also given. Before the recent increase 

 in the sensitiveness of the photographic plates, satisfactory 

 photographs could not be obtained of the spectrum of this star, 

 on account of its red colour. 



I 



INFLUENCE MACHINES} 



HAVE the honour this evening of addressing a few remarks 

 to you upon the subject of influence machines ; and the 

 manner in which I propose to treat the subject is to state as 

 shortly as possible, first, the historical portion, and afterwards 



Lecture delivered at the Royal Institution, by Mr. J. Wimshurst, en 

 April 27, 1888. 



to point out the prominent characteristics of the later and the 

 more commonly known machines. 



In 1762, Wilcke described a simple apparatus which produced 

 electrical charges by influence, or induction, and following this 

 the great Italian man of science, Alexander Volta, in 1775 gave the 

 electrophorus the form which it retains to the present day. This 

 apparatus may be viewed as containing the germ of the principle 

 of all influence machines yet constructed. 



Another step in the development was the invention of the 

 doubler by Bennet in 1786. He constructed metal plates which 

 were thickly varnished, and were supported by insulating handles, 

 and which were manipulated so as to increase a small initial 

 charge. It may be better for me to here explain the process of 

 building up an increased charge by electrical influence, for the 

 same principle holds in all of the many forms of influence 

 machines. 



This Volta electrophorus, and these three blackboards, will 

 serve for the purpose. I first excite the electrophorus in the 

 usual manner, and you see that it then influences a charge in its 

 top plate ; the charge in the resinous compound is known as 

 negative, while the charge induced in its top plate is known as 

 positive. I now show you by this electroscope, that these 

 charges are unlike in character. Both charges are, however, small, 

 and Bennet used the following system to increase them. 



Let these three boards represent Bennet's three plates. To 

 plate No. 1 he imparted a positive charge, and with it he induced 

 a negative charge in plate No. 2. Then with plate No. 2 he 

 induced a positive charge in plate No. 3. He then placed the 

 plates Nos. 1 and 3 together, by which combination he had two 

 positive charges within practically the same space, and with these 

 two charges he induced a double charge in plate No. 2. This 

 process was continued until the desired degree of increase was 

 obtained. I will not go through the process of actually building 

 up a charge by such means, for it would take more time than I 

 can spare. 



In 1787, Carvallo discovered the very important fact that 

 metal plates when insulated always acquire slight charges of 

 electricity ; following up those two important discoveries of 

 Bennet and Carvallo, Nicholson in 1788 constructed an apparatus, 

 having two disks of metal insulated and fixed in the same 1 lane. 

 Then, by means of a spindle and handle, a third disk, also 

 insulated, was made to revolve near to the two fixed disks, 

 metallic touches being fixed in suitable positions. W r ith this 

 apparatus he found that small residual charges might readily be 

 increased. It is in this simple apparatus that we have the parent 

 of influence machines, and as it is now a hundred years since 

 Nicholson described this machine in the Phil. Trans., I think it 

 well worth showing a large-sir.ed Nicholson machine at work 

 to-night. 



In 1823, Ronalds described a machine in which the moving 

 disk was attached to and worked by the pendulum of a clock. 

 It was a modification of Nicholson's doubler, and he used it to 

 supply electricity for telegraph working. For some years after 

 these machines were invented no important advance appears to 

 have been made, and I think- this may be attributed to the great 

 discoveries in galvanic electricity which were made ?.bout the 

 commencement of this century by Galvani and Volta, followed in 

 1831 to 1857 by the magnificent discoveries of Faraday in 

 electro-magnetism, electro-chemistry, and electro-optics, and no 

 real improvement was made in influence machines till i860, in 

 which year Varley patented a new form of machine. 



In 1865 the subject was taken up with vigour in Germany by 

 Toepler, Holtz, and other eminent men. In 1866, Bertsch in- 

 vented a machine, but not of the multiplying type ; and in 1867, 

 Sir William Thomson invented a form of machine, which, for 

 the purpose of maintaining a constant potential in a Leyden jar, 

 is exceedingly useful. 



The Carre machine was invented in 1868, and in 1880 the 

 Voss machine was introduced, since which time the latter has 

 found a place in many laboratories. It closely resembles the 

 Varley machine in appearance, and the Toepler machine in 

 construction. 



In condensing this part of my subject, I have had to omit 

 many prominent names and much interesting subject-matter, but 

 I must state that, in placing what I have before you, many of 

 my scientific friends have been ready to help and to contribute ; 

 and, as an instance of this, I may mention that Prof. Silvanus 

 P. Thompson at once placed all his literature r n.l even his private 

 notes of leference at my service. 



I will now endeavour to point out the more prominent features 



