low rates of flicker are very unpleasant to watch. Early studies by Thomas Edison for his Kinetoscope machine determined that any rate below 46 images per second "will strain the eye". and this holds true for projected images under normal cinema conditions also. The solution adopted for the Kinetoscope was to run the film at over 40 frames/sec, but this was expensive for film. However, by using projectors with dual- and triple-blade shutters the flicker rate is multiplied two or three times higher than the number of film frames — each frame being flashed two or three times on screen. A three-blade shutter projecting a 16 fps film will slightly surpass Edison's figure, giving the audience 48 images per second. During the silent era projectors were commonly fitted with 3-bladed shutters. Since the introduction of sound with its 24 frame/sec standard speed 2-bladed shutters have become the norm for 35 mm cinema projectors, though three-bladed shutters have remained standard on 16 mm and 8 mm projectors, which are frequently used to project amateur footage shot at 16 or 18 frames/sec. A 35 mm film frame rate of 24 fps translates to a film speed of per second. One reel requires 11 minutes and 7 seconds to be projected at 24 fps, while a 16 fps projection of the same reel would take 16 minutes and 40 seconds, or per second. In the 1950s, many telecine conversions of silent films at grossly incorrect frame rates for broadcast television may have alienated viewers. Film speed is often a vexed issue among scholars and film buffs in the presentation of silents today, especially when it comes to DVD releases of restored films, such as the case of the 2002 restoration of Metropolis. Tinting With the lack of natural color processing available, films of the silent era were frequently dipped in dyestuffs and dyed various shades and hues to signal a mood or represent a time of day. Hand tinting dates back to 1895 in the United States with Edison's release of selected hand-tinted prints of Butterfly Dance. Additionally, experiments in color film started as early as in 1909, although it took a much longer time for color to be adopted by the industry and an effective process to be developed. Blue represented night scenes, yellow or amber meant day. Red represented fire and green represented a mysterious atmosphere. Similarly, toning of film (such