Modern magic lanterns; a guide to the ... - Yesterday Image

88 MODBEN MAGIC I/ANTEENB.be made by magnetising a piece of knitting-needle, shouldbe arranged on the top of the block, so that the needle ofthe compasa is just held horizontal when no current ispassing through the^ wire. The magnet should not be toonear or too strong, or the sensitiveness of the galvanometerwill be impaired.i The block being inserted in the lantern,the binding screws connected with the source of the currentby wires, and the compass focussed sharply on the screen,when a current passes the needle will be deflected ; owingto the degree of magnification of the image on the screen,such a galvanometer possesses a considerable degree ofsensitiveness.With the help of the tank shown in Fig. 61, suchexperiments as precipitation, reactions resulting in change ofDiagram showing the Use op an Erecting Prism.colour or appearance in solutions, as well as demonstrationsof capillary attraction, pressures of liquids, etc., can beexhibited. In some of these experiments it is highly desirablethat the image upon the screen shall be the rightway up, while at the same time it is impossible to invert theobjects themselves in the lantern. When this is the casean erecting prism has to be employed. This is shown at A,in Kg. 63. They are very convenient in some cases, butalways entail a considerable loss of hght.The greeting prism consists of a block of glass, having itssides cut to a suitable angle to one another. It is fitted onclose in front of the objedtive, the course of the rays beingthen as represented diagrammatically in the figure. Oneform of erecting prism is shown in Fig. 64 ; this is known as