The Optical Magic Lantern Journal (May 1896)

78 The Optical Magic Lantern Journal and Photographic Enlarger. There are three essential instruments for the purpose :—An induction coil, a battery, and a vacuum tube. The Induction Coil.—_It has been stated on several occasions that effects have been produced with a coil giving only a 2 inch spark, but on enquiry it has usually been found that it was a coil capable of producing a longer spark, but worked at an estimated lower length of spark. To excite a suitably exhausted tube thoroughly well, a 3 inchspark is needful, but it is invariably recommended that a coil should not be continuously worked at its greatest power. If a coil giving a 3 inch spark be used, it must be run at full tension, or the tube will be but poorly excited and the exposure considerably lengthened. Tt will, therefore, be found advisable to have say a 4 inch spark coil, and run it at 3 inches. Many workers are using coils giving a 6 inch or even larger spark, and these work in a considerably more uniform manner at the tension necessary for illuminating the tube than those of smaller size. It will be found advantageous, if the coil is to be used frequently, to be provided with a duplicate set of platinum contact points for the coil. The Battery. —The simplest arrangement consists of Bunsen’s or Grove’s cells, one cell to each inch of spark yielded by the coil; thus, a 4 inch spark coil would need four cells. The trouble of re-charging such a battery is, however, very considerable for other than occasional work, and accumulators will be found far more useful for continuous work. ‘They are supplied to run for a great number of hours, and can be re-charged at any works where a dynamo is employed at a very small cost. ; The Vacuun Tube.—Practically, everything depends on the suitability and efficiency of the tube. The form that was devised by Mr. Jackson, of King’s College, and is obtainable from a large number of dealers, is unquestion| ably the best. Many workers are possessed of this form of tube, but are still unsuccessful in obtaining good results. The fault more often than not rests with the tube, and not with the worker. As a matter of fact, an enormous number of tubes are being sent out which will never do more than produce a shadow effect, and it would be well if all disappointed workers were to make for themselves, or to purchase a fluorescent screen, such as those referred to below, and to test their tubes therewith. There is a difference of opinion amongst workers as to the distance at which the tube should be used from the subject; but it would appear to be pretty generally agreed that 9 to 10 inches is the most suitable. The platinum disc should be as nearly as possible parallel with the subject. ‘What is the life of a tube?’ is a question that is frequently asked. With care there is no reason why it should not last for a lengthy period. Mr. Godfrey Martyn, who was exhibiting at the Crystal Palace, informed me that he had used two of Watson & Sons’ “ Reliable’’ tubes for all his exhibits, which were hourly, for nearly a fortnight without any deterioration whatever, and he is still employing them. Tubes in which the platinum disc gets red hot within a minute of the tube being excited, should be treated with suspicion. They invariably go wrong after very little use. Care should be taken when fitting up the apparatus that the ecmmunicating wires do not touch the glass of the tube. If they are allowed to do so it will almost surely result in irretrievable damage to the tube. If manufacturers could see their way to strengthening the platinum contact ends, to which the wires from the coil are attached, they would earn the thanks of all users of these tubes. Fluorescent Screen.—A large number of materials have been tried with a view to producing these at a low price, but so far with a very small measure of success. It is somewhat ‘ unfortunate that the most effective should be made of platino-cyanide of potassium, the cost of which is about 8s. per drachm. The method of preparation recommended is to coat a thin plate of black celluloid with gum, and allow it to nearly dry; powder the potas slum very finely and mix it up in a thick paste ‘with gum. This should then be painted on the gummed space on the celluloid, allowing suficient time between each coat for drying. The fluorescence is increased according to the amount of potassium that is used. Two firms are supplying these screens commercially, namely Newton & Son, and W. Watson & Sons. I have worked with these screens and found they produced very luminous effects, rendering visible a coin in the middle of a thick book or through a three inch block of wood, the bones of the foot in a boot, ete. Apart from the advantages of the fluorescent screen for enabling bones of the body, etc., to be seen, it is of importance for testing tubes. If the tube produces no luminous effect on the fluorescent screen—and I have met with many taat do not—it is entirely useless for producing negatives, and in proportion to the luminous effect. will be the etticacy of the tube for yielding results and shortening exposure. Every worker should provide himself with a