TRANSACTIONS OF THE SECTIONS. 11 



Mr. Bakewell read a paper on the Copying Electric Telegraph, and illustrated its 

 action by experiments with the instruments. In the method adopted for trans- 

 mitting copies of writing, the letters to be transmitted are written on tin-foil with 

 varnish, so as to present a conducting and a non-conducting surface. The foil is 

 placed on the cylinder of the transmitting instrument, and a metal style in connec- 

 tion with a voltaic battery presses on the surface of the cylinder as it revolves. By 

 this means the electric current is continually broken when the style is resting on the 

 varnish ; and as the style is made to traverse by an endless screw from one end of 

 the cylinder to the other, it passes necessarily over all the lines of the writing, and 

 about eight times over eacii line. The receiving instrument is similar to the trans- 

 mitting one, and on to the cylinder of that instrument paper, moistened with a solu- 

 tion of prussiate of potass in diluted muriatic acid, is placed, the metal style on that 

 instrument being a piece of steel wire. When the electric current from the positive 

 pole of the voltaic battery passes through the steel point to the paper, a blue mark is 

 made by the production of prussian blue, and when the cylinder is in motion the 

 effect is to draw a series of spiral lines on the paper ; but as the lines are broken 

 whenever the varnish writing on the transmitting cylinder interposes, the forms of 

 the letters are transferred from one instrument to the other; the writing appearing 

 of a pale colour on a ground of blue lines drawn closely together. To produce this 

 effect it is requisite that both instruments should rotate exactly together, and this 

 synchronous movement is attained by means of an electro-magnet ; one instrument 

 being made to regulate the other by retarding its motion at regular intervals. The 

 regulation of the instruments is also facilitated by a guide-line, consisting of a strip 

 of paper placed at right angles to the writing, by which means the person in charge 

 of the receiving instrument can ascertain exactly hov/ much the speed of the two in- 

 struments differs, and by the addition or abstraction of weight can bring the gaps, 

 formed by the strip of paper, to fall exactly under each other, which indicates that 

 the two cylinders are revolving at the same rate. It was stated in answer to ques- 

 tions by members present, that two hundred letters per minute might be copied by 

 the instruments exhibited, and that five hundred a minute are attainable. To illus- 

 trate the facility which this means of telegraphic communication affords for trans- 

 mitting secret messages, an apparently blank piece of paper was produced, on which 

 a message had been impressed invisibly before the meeting of the Section, and by 

 brushing it over with a solution of prussiate of potass the writing became instantly 

 legible. 



Remarks on Lord 'Qrowghdims Exiienments on Light, ^c. in the Phil. Trans. 

 1850. Part I. By the Rev. Professor Powell, F.R.S. &)-c. 



The experiments of Lord Brougham " on the properties of light," &c., are re- 

 garded by their author as offering new facts at variance with the principle of inter- 

 ference, hitherto so successfully applied to all pheenomena of this class. They seem 

 therefore to call for some remarks as to their actual bearing on the question. The 

 experiments all refer to the well-known phaenomena of diffraction-fringes formed by 

 the edge of an opake screen ; which the author views in connexion with a peculiar 

 theory odnflecting and deflecting forces ; the nature of the effect being chiefly inves- 

 tigated by placing a second edge at some distance from the first along the ray, and oc- 

 casionally a third ; which produces changes in the breadth and position of the fringes. 

 In the author's attack on the interference theory (especially in prop, xi.), some mis- 

 conception of that theory appears to be involved. Though the undulatory theory 

 has been successfully applied to the general subject of these fringes, yet it is well 

 known that the application of the formulas to any but the simplest cases of edges 

 and apertures is defective, owing to the great complexity of the resulting expres- 

 sions, and the impossibility of integrating them except under very restricted condi- 

 tions. Thus the integration has not been extended to the action of a second or third 

 edge at different distances; this last case being obviously the same as that of an 

 aperture or screen whose plane is inclined to the path of the rays. Fresnel, in his 

 justly celebrated memoir (Sur la Diffraction de la Lumiere, Mem. de I'lnstitut, tome v. 

 for 1821, published in 1826, note, p. 452), considers Irieflij this very case, and shows 

 generally that the fringes will not be symmetrical, having a greater extension towards 

 one side ; but he does not give any analytical investigation, which would manifestly 



