1212 



Megadiascope. 



Spectrum Phenomena with a direct-vision prism. 



Fig. 12. 



D = Direct-vision prism, C = Collimator lens, 

 F= Adjustable slit. 



8. Demonstration of Spectrum Phenomena 



(Fig. 12). 



An adjustable slit (Fig. 12), F, is placed on the optical 

 bench in front of the condenser. At some distance from 

 the slit the collimator lens, 4, is set up and adjusted until 

 a sharp image of the slit is obtained on the screen. The 

 direct-vision prism, D, is last of all introduced in the passage 

 of the rays, thus producing an extended spectrum on the 

 screen. The slit has a small piece suitably arranged for the 

 reception of an absorption vessel. 



9. Demonstration of all Polarisation Phenomena in Parallel and in Convergent Light (Figs. 13 19). 



Polarisation phenomena may be demonstrated 

 either with the Projection - Polarisation apparatus 

 (F i g. 13) or by Paalzow's method in the open (F i g. 16). 



The Projection-Polarisation apparatus (F i g. 13) 

 consists of a wooden box in which is contained a column 

 of glass plates. The box has a large aperture diaphragm 

 at the side opposite to the condenser of the megadiascope; 

 at the reverse side it is closed by a sheet of brass to the 

 opening of which a funnel is -screwed. This funnel has 

 a lens at the wide end, and at the narrow end it is provided 

 with a thread into which the neck of the nicol screws. 

 A special holder is fitted at the front of the apparatus, 

 which is intended to take the neck of the prism when it 

 is screwed off the funnel. 



The polarisation apparatus is used in two different 

 ways (1) for the projection of preparations by means of 

 parallel, or more correctly, weakly converging light-rays; 

 (2)'~tor polarisation in strongly convergent light. 



Projection Polarisation Apparatus. 

 Fig. 13. 



Polarisation in Parallel Light. 

 Fig. 14. 



Polarisation in Converging Light. 

 Fig. 15. 



To project in parallel light, the funnel is screwed off from the box and on to 

 the neck of the nicol, and the objective head of the megadiascope is placed in front of the nicol (Fig. 14). 

 The preparations are contained in a holder, the two being placed in the space intervening between 

 the box and the funnel. The arc lamp is adjusted in relation to the mirror in such. manner that the 

 light-pencil issuing from the condenser is longer, and the light-rays thus pass from the condenser slightly 

 convergent. The following are quite suitable for projection in parallel light: rapidly annealed glasses, 

 thin slabs of gypsum, and gypsum figures. 



To avoid any serious heating of the nicol it is well to extinguish the arc lamp at the intervals 

 between the introduction of the various specimens, or to shut off the polariser by means of the shutter 

 supplied with it, so that the light-pencil does not rest too long on the nicol prism. 



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