In the 1930s slideshows were greatly improved by the invention of the Kodak slide projector and 35mm slides. Slideshows were still mostly presented to large audiences, however. They were often used for educational purposes, for teaching art history. Slides were also used at the cinema sometimes to advertise local businesses or to request good behaviour from the audience,gentlemen would be asked to remove their hats. The invention of the carousel projector in 1961 made these slideshows even more popular. This projector enabled many slides to be placed on a carousel. The slides could be changed by using a single button on the machine. Carousel projectors were also bought by companies and education institutions for presentations. Dual carousel projectors were often used for presentations because of the high quality of image resolution and to enable the presenter to show visual comparisons of images.