^ REPORT — 1870. 



On an Improved Lantern for Lecture demonstrations with Electric Light. 



By W. Labd, F.E.A.S. 



The body of the lantern is formed of a brass tube 9 in. diameter, in the centre of 

 which is fixed the electric light _; at two points in the tube, situated about 100° apart, 

 are openings, to one of which is attached a tlange into which the various optical 

 arrangements slide ; the other opening has a sliding piece caiTviug a second and 

 similar flange, so that the various pieces of apparatus are mutually interchangeable : 

 by pointing one of these openings direct on to a screen, the other will be in the 

 proper position for a ray of light passing through two bisulphide-of-carbon prisms 

 to give the spectrum on the same screen ; by this arrangement the demonstration 

 of the arc and then of the spectrum of any metal becomes one of great simplicity ; 

 for without altering the position of any part of the apparatus, and the metal still 

 continuing to burn, you simply have to uncover the opening, which gives you the 

 arc ; then closing this and uncovering the second opening, immediately will appetu- 

 the spectrum of the same substance. Attached to the body of the lantern is a very 

 useful adjimct to the instrument, consisting of a small sh ding-tube, to one end of 

 which is fixed a piece of dark ground glass and to the other a lens ; these are so 

 adjusted that a perfect image of the carbon-points can be seen by the operator ou 

 the dark glass without openmg the lantern or admitting stray light into the room ; 

 and by having a line ruled in the ground glass central with the various optical 

 arrangements and simply watching the image there produced, the lamp can be so 

 adjusted as to keep the light absolutely central — a desideratum every one will admit 

 who has had to use a microscope or polariscope with the electric light. Within 

 the lantern is fixed a small gas jet to enable the operator readily to change tho 

 carbon-points, &c. 



The shifting-motion of the flange is, for the pui-pose of adjusting the angle of the 

 openings, to suit the different distances of the lantern from the screen ; and as one 

 lantern is all that is required, there is very little obstruction to the view. 



On a New Absolute Electrometer. 

 By Professor Sir William Thomson, M.A., LL.D., F.B.8. 



On a new Field of Magnetic Besearch. By Tkedebick H. VAELEr, F.B.A.S. 

 A permanent magnet was exhibited, which displayed the action of magnetism 

 upon the crystallization of iron. The author referred to this fact as affording a 

 means for the futui-e investigation of magnetic phenomena, and also showed that 

 diamagnetic substances could be crystallized in magnetic fields, and thus would 

 define permanently the action of magnetism on the crystallization of magnetic and 

 diamagnetic metals. He expressed his belief that, ii-om experiments commenced 

 in the year 1858 and others conducted in 1870, electricity and magnetism con- 

 joined would produce permanent magnetic structures, shovsong in a solid form the 

 lines and nature of magnetic force. 



On a Constant Battery. By Fredeeick H. Vaelet, F.B.A.S. 



The author exhibited a new form of batteiy, designed by Mr. Octavius Varley, 

 for the purpose of removing the defects of the otherwise constant battery of the 

 late Professor Dauiell. In this form of battery the porous chamber, which has been 

 the som-ce of all the trouble, is removed. The battery comprises a water-tight 

 compartment for the copper-salt, a connecting column of fluid, and a compartment 

 for tbe zinc-salt. These are so arranged that, whilst the electro-chemical con- 

 tinuity is completed at the top of the copper-chamber, no intermixture of the salts 

 can take place ; convection-cm-rents, which are the great source of intermixtm'e, 

 are divided into three separate systems, which cannot possibly interfere one with 

 another. Whilst economy of the copper-salt is effected, greater uniformity and 

 higher potential is maintained. The zinc plate is found to work out to the very 

 last, bemg kept free from the coating of copper to which aU batteries hitherto 

 constructed are liable. 



