Bauhaus-Universität Weimar

light in the lecture-room does no harm and is par¬ 
ticularly useful in maintaining the relations between 
speaker and audience and permitting the students to 
go on taking notes. 
The smooth progress of the lecture and the in¬ 
structor’s peace of mind are greatly enhanced by the 
fact that the assistant can go on preparing the next 
projection by daylight or artificial light in the next 
room. If the screen is arranged (Fig. 1) somewhat 
ter the side of the desk so that no direct light from the 
side windows in the lecture-room can reach it, and if an 
opaque oilcloth screen, attached like a window shade, 
with its outside surface whitewashed, is mounted 
behind the transparent screen, the operator can throw 
an image visible to himself and to the instructor but 
not to the students on this screen. If the lecture-room 
is connected with the side room by a wide opening, 
the instructor can give the operator instructions in a 
low voice or by unobtrusive signals without thereby 
distracting the students, who can see nothing behind 
the screen, and he can even, if necessary, walk over to 
the machine and make any necessary alteration with¬ 
out a more than momentary interruption of his lecture. 
Only when the image is found satisfactory and when 
it is needed in the lecture does the speaker raise the 
roller curtain behind the screen and thus disclose the 
now transparent image to the class. If the screen is 
large enough one can even project two images at once, 
one above the other, and show them one at a time and 
then together for purposes of comparison. As soon as 
the speaker has no further need for the pictures he 
pulls down the shade and thus wipes them out on the 
front of the screen with a single gesture. 
If the students need to copy tables, formulae, or 
sketches, they can do so after the lecture, the picture 
then being left on the screen as long as necessary. Such 
material can easily be copied on old washed photo¬ 
graphic plates of slide size, which does away with the 
time-consuming habit of writing all this on the black¬ 
board and makes it quite certain that the calculations 
will be correct, as they may not be when written on the 
blackboard in a hurry. For regularly recurring lec¬ 
tures this kind of material, copied from books, can 
be made into a single numbered series of slides and 
kept in a drawer. Such collections have the ad¬ 
vantage that they take up very little room and can 
always be kept up to date at small expense, especially 
if the institution has a camera of its own. Expensive 
chart material thus becomes entirely superfluous, and 
this compensates to some extent for the large initial 
cost of the projection machine. 
In Figure 1, A represents the projection machine, B 
the screen, and SS' small sunken tracks along which 
both the machine and the stand on which the screen is 
supported can slide. These tracks insure that the 
optical axis of the machine will always remain per¬ 
pendicular to the surface of the screen—an absolutely 
necessary requirement for clearness of all portions of 
the picture—and at the same time make it possible to 
alter the distance between the machine and the screen 
in order to alter the size of the image. Both the 
machine and the screen are mounted on wooden frames 
running on four rollers each, the pair on one side being 
in each case fitted to a flat track, while the opposite 
pair serve as guiding wheels and have an obtuse- 
angled edge fitting into corresponding grooves in their 
track. Both tracks are sunk into the floor to avoid 
forming an obstruction. 
Since the screen is movable it can, if necessary, be 
brought closer to the spectators, and, most convenient 
of all, the instructor can himself give the screen a slight 
kick that will regulate the sharpness of the picture 
without the intervention of the operator. This is 
especially useful when moving or perhaps living objects 
are being projected, and does away with the need for 
calhng, “Better focus, please,” “Not right yet,” etc. 
Electrical connections that will not interfere with the 
handling of a movable machine of this kind or other 
activities are achieved by twisting the wires together 
and running them over two pulleys in the ceiling to 
the center of the tin roof covering the machine, where 
they connect with the lamp cord by means of a screw 
plug. The pulleys are mounted on swivels, and a free- 
hanging pulley is hung on the wires between the original 
socket in the ceiling and the first pulley, thus taking up 
the slack necessary for a movable apparatus and pre¬ 
venting the cable from getting in the operator’s way. 
When the screen is at one side, as shown in Figure 1, 
it is not placed parallel to the wall, but at an angle of 
22° towards the center, so the tracks and the machine 
have to be placed at the same angle. The rear windows 
in the lecture-room are darkened with opaque curtains 
during projection, and the large window in the side 
wall is provided with draw curtains that the instructor 
can manipulate by means of cords from his desk, 
closing them only as much as he wishes. A shutter 
arrangement composed of movable slats is built into 
the center of this window, so that the amount and 
direction of light admitted can easily be regulated, 
no direct fight being allowed to strike the screen. 
Artificial illumination is provided by three rows of 
fights in the ceiling, with funnel-shaped shades, all 
of them controlled by switches at the instructor’s desk, 
but it is usually unnecessary to turn off more than the 
two front rows. 
If a new building is being erected, all the construction 
best adapted to this kind of projection should be in¬ 
cluded in the original plans. The arrangement de¬ 
scribed meets all the requirements we have laid down


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