1206 



Megadiascope. 



Megadiascope, small Model, the arc lamp fitted with automatic Regulator. 



Fig. 4. 1 :'lU. 



For the projection of microscopical preparations the projection microscope (Fig. 7, 7 a 

 and 7 b) is employed. 



We will now give a description of the foregoing apparatus, also others for demonstrating the 

 spectrum, polarisation, and interference phenomena, the projection in natural colours (after Ives). 

 The arrangements of the experiments will also be described. 



Description of the Individual Experiments, 

 a) Employing transmitted Light. 



I. Projection of Diapositives (Fig. 4). 



Two grooved rails are fitted in front of the condenser, into which former the changing 

 frame for the diapositives can be slipped. The optical bench is drawn out from under the 

 apparatus and the objective earner is placed upon the bench. The light falls from the crater of the 

 positive carbon of the arc lamp on to the parabolic concave mirror, and is thence reflected as a parallel 

 pencil on to the condenser. The distance between the arc lamp and the parabolic mirror can be adjusted; 

 in the larger apparatus the mirror can be brought nearer to or further away from the condenser, 

 after loosening an adjusting screw, by pushing in or pulling out a knob which is placed at the end 

 of the megadiascope on the side opposite to the condenser, the arc lamp remaining stationary. In the 

 small megadiascope, however, the arc lamp is moved to and fro by means of a screw fitted witli 

 handle, fixed in the corresponding position of the apparatus: the parabolic mirror remaining stationary 

 during this operation. By this means the light can within certain limits be concentrated more or less 

 on the condenser with the object of increasing the brilliancy of the image. 



With a Petzaval projection objective of 120 mm. focal length, a sharp and brilliant image of 

 3x3 m. is obtained from a diapositive 7x7 cm. the distance between objective and screen 

 being 4,5 m. If it be desired to obtain at the same distance images of the same size from larger dis- 

 positives, objectives of correspondingly longer focus should be used, and should be ordered as an extra. 

 Eegarding the size of image at other distances reference should be made to p. 1204. 



Sometimes it is desirable that the centre of the image on the screen should be higher than 

 the centre of the objective on the megadiascope, e. g., in projecting over the lecture table. To 

 facilitate this, the superstructure of the megadiascope can be raised by means of a vertical 

 screw with handle fitted to the substructure at the objective side. The pencil of light is thus directed 



Cl 4662. 



