Projection Apparatus and Accessories. General. Source of Illumination. 149 



Projection Apparatus and Accessories. 



General. 



When installing a projection lantern for the purpose of teaching physics, it is of importance 

 to apply to a firm manufacturing physical apparatus in general. A great many points enter into 

 consideration in the proper arrangement of an apparatus employed for projection in connection 

 with scientific subjects which only the manufacturer of apparatus for teaching physics 

 thoroughly understands. The projection lanterns constructed by us fulfil every conceivable 

 requirement; they are constructed in such manner as to permit not only of the projection of 

 lantern slides, but of the projection of horizontally placed transparent objects, of opaque objects 

 (e. g., illustrations from magazines, drawings, photographs, flat objects); also the projection of 

 microscopical preparations. In addition it is possible to obtain animated pictures with the cinemato- 

 graph; and, in conjunction with an optical bench, polarisation apparatus, etc. it is possible to demon- 



>> strate many experiments in connection with the teaching of light, spectrum experiments, polarisation, 

 interference, and diffraction phenomena, etc. Moreover, the projection lanterns are arranged that 

 it is only necessary to place in front of the lantern the apparatus dealing with the various branches of 



J> physics which it is intended should be projected; in this connection mention may be made of Plateau's 

 apparatus for showing the oblateness of a rotating spheroid of oil; the apparatus for showing total 



,g reflexion in a water jet, the apparatus for demonstrating that a jet of water is composed of drops, de- 

 monstration of electrolytic decomposition, Kolbe's electrometer, etc. It is this latter method of using 

 the lantern which renders it of such value for teaching. The apparatus necessary for the individual 



g experiments, together with some instructions, are included later on in the list, and we refer our custo- 

 mers to these and to the large selection of apparatus which are listed in the main catalogue. The pro- 



O jection apparatus which are intended solely for the projection of diapositives are specially indicated. 



Source of Illumination. 



% Selection. Without a doubt the electric arc light is the most suitable for projection purposes, 



S possessing a large number of advantages over the other kinds of illumination considered. The Nernst 

 lamp has found its way into use in recent times along with the arc lamp. In view of the present state 

 and continued rapid development of things electrotechnical, it is advisable either to use a Projection 

 Lantern having an electric arc lamp or else one which in addition to being lighted by electricity, can 

 also, if desired, be supplied with some other source of light. The first case will occur most often, 

 for at the present time most schools in small towns have electricity available by being connected with 

 a Municipal or private generating station. Even in places where this source of supply is not at hand 

 at present it might certainly be well to have at the commencement a lantern which can be adapted to 

 electric light. As electric generating stations are rapidly springing up in every direction, it may be 

 taken as a certainty that electricity will within a short time be available for supplying the lantern. 

 As a matter of fact a great many small townships possesses to-day electricity stations which were 

 thought quite impossible a few years back. If no source of electricity is available, the following 

 principal sources of light must be considered: Lime Light, Thorium Light, Spirit, Petrol, Paraffin 

 and Incandescent Gas, and also Acetylene light. 



Electric Arc Light. The following principal advantages may be set forth in favour of this me- 

 thod of illumination: It is simple to attend to; the carbons burn for a number of hours so that renewal 

 is not often necessary. The lantern is connected up to the supply by a flexible cord, the current being 

 switched on and off by a switch. Cleanliness is another strong point, as no substances are used which 

 cause soot, or which corrode, cause the formation of grease, or which produce a strong smell. The safe- 

 ty in working is great; materials which may cause tire or explode are done away with; and all risk 

 both to operator and experiments is obviated. In view of the high illuminating power of the arc lamp 

 (employing direct current this is about 1500 Hefner candles with 15 amps., 2200 H. C. with 20 amps.; 

 3000 H. C. with 25 amps.; and employing alternating current, about 450 H. C. at 15 amps.; 700 H. C. 

 with 20 amps.; and 800 H. C. with 25 amps.) the scope of lanterns with arc light is very considerable; 

 a number of experiments can be carried out at high powers of magnification as well as projections with 



