Preparation for a conference or a seminar is not a trifle. One has to marshal information, write and correct the text, prepare figures and plots, and ultimately make up a presentation of all that.
Today it is hardly believable, but there is more to it. To be more exact, everything was different after the list item “Marshal information”.
This is because the text had to be handwritten or given to a typist for typewriting, and it was edited using a correction fluid, and formulas were inserted by hand. Figures were made in the form of slides and shown with a slide projector.
To make plots was even a harder job, since they had to be copied on page-sized sheets of special transparent film with rows of formulas and handwritten text lines added there.
Demonstration of images on these transparent sheets required a special device, an overhead projector (OHP). While talking, the lecturer changed the film sheets projected on a large screen. During discussions, if any, formulas could be written on immediately available clean transparencies and demonstrated to the audience.
Overhead projectors were widely used at seminars and lectures in the 1970–1990s.
Photo: transparencies from the Museum-Study of Bruno Pontecorvo; seminar of Prof. G. Bayer at LNP, JINR, Dubna, 1992 (JINR Photo Archive).