156 Projection Apparatus and Accessories. 



lantern is arranged horizontally, the image will fall too low on the screen since the lantern cannot well 

 be placed higher than is convenient for purposes of manipulation. In all these cases it is necessary 

 to have the lantern arranged tilted, so that the image is proportionately higher and falls on the centre 

 of the screen. For tilting, either a table top which can be sloped (No. 50,993) is used, this being simply 

 laid on the top of the lecture table; or recourse should be had to a stand (Nos. 50,995 et seq.) with top 

 which can be tilted, the projection lantern being placed thereon. The projection screen must be cor- 

 respondingly inclined so as to obviate any distortion of the images. The screen can be very simply 

 tilted by having two weights at the right and left of the screen and a little behind the same. Two cords 

 should be carried from the weights to the weighting bar of the screen, and the screen itself can thus 

 be regulated so as to assume the correct angle of inclination. 



The screen itself should best be made of an opaque dull white material; the most suitable size 

 being in most cases 3x3 metres. Transparent screens need only be considered where the lantern must 

 be installed in a room behind the screen. For a few interference, polarisation and diffraction experi- 

 ments it is of value to have a special small and handy transparent screen which can be set up near the 

 lantern. The phenomena are then strongly illuminated and the scholars can pass by behind the screen 

 in rapid succession. 



For rolling the screen up and down a draw cord device is used, or a rolling device, driven by 

 electric motor, may be employed. This latter arrangement can be set into motion from any point in 

 the room simply by putting the motor into circuit by the aid of a hand reversing switch, which, along 

 with the fuses necessary, is fixed to a marble slab on the wall. The motor is automatically put out of 

 gear as soon as the screen is completely rolled up or down. 



The room is best darkened by means of light-tight blinds which can be rolled up or down; 

 this arrangement can be worked either by hand or by motor. PP. 5 and 29 32 contain all particu- 

 lars as to these devices. When asking for prices for darkening arrangements, kindly always forward 

 accurately dimensioned sketches if possible, send constructional plans, as the prices depend essen- 

 tially on the size, number and arrangement of the windows. 



Uses of the Projection Lanterns. 



Projection of Diapositives. 



The projection of diapositives is utilised as an important addition to instruction in botany, 

 zoology and mineralogy, physical geography, geology, cosmology; but more particularly for nearly 

 all branches of physics and chemistry; in addition the art of projecting plays a part in the teaching 

 of history, the history of art, and religion. The advantages of the projection lantern therefore come 

 specially into play when they are ready installed for use on the spot, and when the class room can be 

 rapidly darkened, this being rendered possible the most advantageously with electric projectors and 

 suitable darkening devices. It is also advisable to have the room lighted by incandescent lamps. The 

 diapositives are laid in the change frame, the objective being then adjusted first in a coarse manner 

 and subsequently in a fine manner until the image is focussed sharply on the screen; after this it is 

 possible to go on changing the pictures in regular order. 



As a rule the projection lanterns are supplied ready complete for the projection of diapositives; 

 it remains for the photographs themselves to be provided. We usually supply these in size 9x10.5 cm. 

 If it is required to project different sized pictures, as 8.5x10 cm or 9x12 cm, special change frames 

 are provided for the purpose, these being supplied, at the same time as the lantern, for the 3 sizes named. 

 A diapositive holder with change frames for plates 13 x 18 cm (No. 50.973) can be used with all lanterns; 

 it must, however, be borne in mind that, especially with small condensers, only a small portion of a 

 picture 13 x 18 cm can be cast on to the screen. This holder with change frame for 13 x 18 cm photo- 

 graphs must be specially ordered when required, and is charged for as an extra. 



Projection of Translucent Physical Apparatus. 



Many physical phenomena occur in so minute a form as to make it impossible to follow them 

 at all with the naked eye, and others can be observed by only one person. In these cases, when a pro- 

 jection microscope is not available, the lantern is the only means of rendering the phenomena simulta- 

 neously visible to a large audience. Only a few of the experiments will be mentioned here: 



The decomposition of a jet of water into drops by means of a stroboscopic disc; capillary 

 phenomena; wave-projection machine; Newton's transparent colour discs; sectional model of a 



