158 Projection Apparatus and Accessories. 



By the aid of a Colliinator Lens (No. 50,982) a sharp image of the slit is projected on the screen. 

 A prism -- preferably a direct vision prism --is placed on the Prism Stage, the latter being brought 

 into the narrowest part of the light pencil issuing from the collimator lens, the result being a sharply 

 defined spectrum on the screen. The direct vision prisms have the advantage over the prisms with 

 deflecting ray, for projection purposes, as they obviate the tilting and lateral adjustment of the lan- 

 tern. In addition, Wernieke and Konigsbcrger liquid prisms may well be considered. These prisms 

 give specially great brightness of image; the liquid may be kept in the prisms. For determining and 

 comparing the refractive capacity and the different dispersion of liquids, use is made of the carbon 

 disulphide prisms and also of the hollow prisms, these being catalogued in a large variety in the section 

 of this list dealing with Optics. Opportunity offers here for mentioning also the reproduction of the 

 solai 1 spectrum by using the transparent solar spectrum. The fact that a spectral colour can be no further 

 split up by a prism can be confirmed by placing a slit behind the prism, whose aperture is then illu- 

 minated with the desired. colour, after which a further prism is placed in the light pencil. For this ex- 

 periment an adjustable Slit, on stand, as well as a small stand containing a special prism, can be em- 

 ployed. The second slit is left fairly wide (up to 5 mm or thereabouts) and is set up at not too great 

 a, distance from the projection screen. 



The resolution of the colours of the spectrum into white can be shown by the aid of the appa- 

 ratus listed in the optical section of our list; in this apparatus the individual colours of the spectrum 

 projected on to a number of mirrors are projected from these on to a separate small screen or a white 

 pasteboard slab (placed at right angles to the large screen) by focussing the mirrors on one point: a 

 white spot resulting from the mixing of all the colours. This experiment is especially instructive be- 

 cause the mixing of the colours can be carried out in full view of the students, and the mixing of the 

 individual complementary coloursand other colours can be conveniently effected before or afterwards. 

 In setting up the apparatus with 5 or 7 mirrors in front of the lantern, these mirrors should be at an 

 angle of 45 to the optical axis, but care should be taken that those mirrors encountered by the feebler 

 part of the spectrum are nearest the lantern so that they take up a larger angle of rays than the others. 

 An Oscillating Prism is used to demonstrate the theory that the actual colours of the spectrum ap- 

 pearing on the same spot in rapid succession appear white; we supply this 'prism to suit the whirling table. 

 The prism, together with the driving device, is set up in front of the lantern in such manner that the 

 prism itself is arranged as an ordinary prism while the spectrum is being projected, being afterwards 

 set in motion. Even imitated spectrum tints give white when reproduced additively. This experiment 

 can be made in conjunction with the previous ones by employing the transparent colour discs. Another 

 method for re-uniting the colours of the spectrum consists in inserting an achromatic (spherical) lens 

 in the path of the rays behind the prism; a cylindrical lens may even be used for the purpose. 



Various emission spectra can be projected with the lantern in rapid succession, by the aid of 

 carbons filled with the salts of various metals. For rapidly changing the carbons a revolving arran- 

 gement is most advantageous. In these experiments the current must be passed through the lantern 

 in the reverse direction to that which is usual, i. e., it must be led in at the lower carbon and out at 

 the upper. The experiment with sodium salt must always be made last, or the spurting of the sodium 

 will cause the other spectra of the sodium line to show. 



A small bench is arranged on the adjustable slit on which may be placed an absorption trough, 

 e. g., for liquids or gases, for the purpose of carrying out absorption experiments; this little bench can 

 also take a colour slab or the like. The liquid specially adapted for absorption experiments is a diluted 

 solution of potassium permanganate. The slit with iris diaphragm (No. 50,988) mentioned above con- 

 tains special springs for clamping the slabs or coloured glasses or other preparations. For reversing 

 the sodium line the apparatus suggested by Frankland is used. This is placed between the slit and 

 the collimator lens in such manner that the pencil of light must pass the flame, a pure spectrum being 

 projected on the polarisation screen. If a small piece of sodium the size of a pea be now put into the 

 small platinum spoon of the apparatus, thus colouring the flame an intense yellow, the sodium line 

 shows black on the projection screen. 



For other spectrum experiments the prisms constructed of various sorts of glass, liquid prisms, 

 compound and crossed prisms, included under Optics, are employed. 



Interference and Diffraction. The type of screen used is the small transparent screen No. 51,003, 

 as the phenomena are somewhat weakly illuminated and it is necessary that the scholars should view 

 them individually. By employing this screen the scholars can pass by behind the screen without dis- 

 turbing the rest of the class. 



The interference prism is set up in front of the optical bench, and the bi-concave lens and the 

 adjustable slit on the bench itself, and the small projection screen is brought up to the lantern until 

 the interference bands are sharply focussed on the screen, which may subsequently be removed. For 

 demonstrating the diffraction phenomena the slit No. 50,985, adjusted to a width of slit of 2 mm, is 

 placed in front of the condenser and a second slit, say, adjustable slit No. 50,986, is placed on the op- 



