Megadiascope. 



1213 



ts 

 a 



ts 



a 



c. 



o 

 M 



x 

 a 



The fourfold change of the phenomena is brought about by rotating the neck of the prism, 

 and "not the specimen. 



For projecting in strongly converging rays, the funnel is screwed off the neck of the nicol and on 

 to the box. The preparations are held in the space intervening between funnel and nicol neck (F i g. 15). 



The objective head of the megadiascope is not employed in this arrangement; the remarks 

 made above as to the adjustment of the arc lamp apply in this case also. 



The following are well suited for projection in strongly convergent light: calc-spar, rock crystal, 

 aragonite. potassium cyanide, strontium aceto-cuprate and tourmaline. 



The Open Arrangement after Paalzow (F i g. 16) has the advantage that all parts necessary 

 for polarisation, such as nicols, lenses, condensers, holders for the preparations, etc. are mounted indi- 

 vidually in the open so that the course of the rays may be followed. The nicol mounts lie in half 

 round bt-a rings from which they can be quickly and easily removed. In this way it is possible for the 

 phenomena to be demonstrated rapidly one after the other first in polarised and then in unpolarised light. 



P O H N L 



Polarisation in Converging Light with 2 Nicols and 2 Condensers 

 for uniaxial and biaxial Crystal*. 



Fig. 16. 



Polarisation in Parallel Light with 1 nicol as Polariser 



and 1 double refracting Prism. 



P = double refracting Prism, O = open Objective. 

 H = rotating object holder. ,Y - Xi?ol, L = bicon- 

 cave lens. 



Fig. 17. 



Fig. 16 shows the open arrangement 

 for polarisation in converging rays. In front 

 of the condenser is first placed a bi-concave 

 lens which renders parallel the converging 

 rays issuing from the condenser. The parallel 

 pencil now passes successively through the 

 large polarising nicol, the first condenser, 

 the preparation, the other condenser, the open 

 objective, and finally the small analysing nicol. 

 The images of the axes even of biaxial crystals 



appear on the 4 m. distant projection screen I 1 ;., 2 m large and in a very beautiful manner. 

 The qiiadruple alternation of the phenomena is obtained by rotating the analyser. 



Fig. 17 shows the arrangement for polarisation in parallel rays. Instead of the nicol prism 

 an achromatic, double-refracting calc-spar prism is employed as analyser, in order to demonstrate 

 that such a prism may be used both as polariser and analyser. 



S O H G L P O H L 



Polarisation with Glass Plate Column and dark Mirror. 

 S= dark Mirror, O = open Objective, H = rotary Object 

 Holder, G = Glass Plate column, L = bi-concave lens. 

 Fig. 18. 1:10. 



Double Refraction with 1 or 2 double refracting prisms. 

 P= double refracting prism, O = open Objective, //= rotary 

 Object Holder, L = bi-concave lens. 

 Fig. 19. 1 :10. 



Fig. 18 illustrates the arrangement with Glass Plate Column and dark Mirror. Fig. 19 

 shows double retraction with 1 or with 2 double refracting prisms. 



Cl. 4667. 4917, 191S.4920. 



