1204 



Megadiascope. 



brilliancy of the image is obtained with a proportionately low current consumption, and opaque drawings, 

 illustrations and flat objects are projected by reflected light with the requisite difinition and brilliancy. 

 The brilliancy of the views resulting from the projection of opaque book 

 illustrations is of course dependent on the candle-power of the source. When 

 importance is attached to great brilliancy of image, it is advisable to employ the large m e g a d i a- 

 s c o p e with 50 amp. arc lamp. The large megadiascope fitted with 30 amp. lamp or even the small 

 megadiascope with 25 amp. 1 ) lamp will probably suffice for less exacting demands. 



The direct current arc lamp can be supplied either with automatic or hand regulator. 

 More recently the preference has been given to the hand regulator on the score of its requiring little 

 attention, being more certain in its action, and somewhat less expensive. The automatic regulators 

 require no attention and are very reliable. See Figs. 2 and 3. 



Size of Image. Projection by transmitted light is effected by an objective of small focus, while opaque objects 

 projected by reflected light an objective of larger focus is used. The reason for this is that the diaposi- 

 tives ordinarily obtainable have an opening of image of 7 x 7 cm. and require to be magnified 40 times 

 if the size of the resultant image on the projection screen is to be 3x3 m. The illustrations to be 

 projected should on the other hand, be as large as possible, since when these are magnified more than 

 20 times the images are not brilliant enough. If it be desired to carry out both systems of pro- 

 jection with a long focus objective, it will be then be necessary to alter the position of the apparatus, in 

 order to get on the projection screen equally large images of the unequally large objects; e. g., with 

 the small diapositives of 7 x 7 cm. free opening of image, the distance of the megadiascope from 

 the screen would have to be 13 m. while in the case of drawings 17 cm. in diameter a distance of 4 to 5 m. 

 from the screen would be correct. In many cases a variation of the distance in these limits is quite 

 out of the question as the class rooms and lecture theatres are amphitheatrical in construction, thus 

 rendering it impossible, or at least very inconvenient, to move the projection apparatus backwards 

 and forwards. In the arrangement with two objectives of different foci the position of the megadiascope 

 can remain unaltered, and images of sufficient and equal size are nevertheless obtained. 



When for certain reasons it is desirable to place the megadiascope at a greater distance than 

 4 to 5 m. feet from the screen, objectives of longer focus must be selected to allow 

 of the image appearing on the screen not more than 3X3 m. with consequent weakness of illumination. 

 The following table explains the various ratios. 



Table of Distances between Megadiascope and Screen, of Size of image appearing on the Screen and as 



to the focus of the Objectives. 

 Free aperture of the Diapositives 8,5x8,5 cm.; size of the opaque images 17 cm. 



Description of Kohl's Megadiascope. 



Fig. 1 on the title page shows the large model Kohl megadiascope 1/15 actual 

 size; the apparatus is erected on a portable table. F i g. 2 shows (1/20 actual size) the smaller model 

 which is likewise erected on a portable table. An illustration of the smaller model with hand regulated 

 arc lamp is shown in Fig. 3. The substructure of the smaller model has a small cupboard 

 for containing the horizontal projector, megascope, etc. 



The superstructure of both models is fitted with a sheet iron house containing the 



') In the case of a 3 wire system a current of 25 amperes can only be taken from the outer and inner leads. 

 A proportionately smaller and not more costly series resistance is therefore necessary and working is not more 

 expensive on account of the nullification of an unproportionately large excess voltage. 



