"MK-2
The main observation devices of the MK-2 are observation slits and mirror periscopes.
A dual observation slit and a tilting all-round reversible periscope are available in front of the driver. In addition to the sights, the crew in the turret has a periscope. The commander's cupola has a periscope and an observation slit. The observation slits are not protected with glass. The driver has a celluloid shield to protect him from dirt. The periscopes are the standard rotating tilting type. The angles are given in figure 9.
Figure 9: MK-2 tank. Dead zones: driver - 3.8 m (slit), 3 m (mirror periscope), turret crew - 11 m (mirror periscope), 13.5 m (commander's cupola).
The MK-3 (Valentine) tank has observation slits and mirror periscopes. The driver has two rotating reversible periscopes. There is a double slit in the entrance hatch covered with a celluloid shield from the inside. Two rotating tilting periscopes are installed in the turret (figure 10).
The periscopes allow for good observation in all directions. The slit cannot be used in battle as it is does not have protective glass.
Figure 10: the MK-3 tank. Dead zones: driver - 3.7 m (slit), 3 m (mirror periscope). Turret - 11.5 m (right periscope), 13 m (left periscope).
The complex shape of the hull and large crew necessitate a large amount of observation devices. One tilting mirror observation device is installed in front of the driver. Two devices are installed in the left and right hatches of the turret platform. The same observation devices are installed in front and to the left of the gunner and to the back and to the right. Observaion devices in the hatches and the rear observation device are rarely used.
There is one mirror observation device in the turret in addition to the sight. The commander's cupola has a sight and two observation slits in the sides with protective glass.
Despite the large number of observation devices, there are many dead zones (figure 11). The driver's vision is unreliable as he does not have a backup observation device.
Figure 11: M3 Medium tank. Dead zones: driver - 2.3 m (slit), turret platform - 6.6 m (observation devices), turret - 10 m (observation device).
M4A2
The driver has the following observation devices: a slit pointing forward, covered with a shutter from the outside and with two layer protective glass from the inside. A rotating mirror periscope is installed in the hatch above the driver. It is installed in a replacable cartridge and can rotate and tilt.
The hull gunner to the right of the driver has the same observation devices as the driver. There is a mirror periscope in the turret to the right of the gun that doubles as a gun sight. The same sight is installed on the left, in the entrance hatch. The mounting slot allows the sight to turn and tilt.
There is an observation hatch in the rear left of the turret. The placement and range of observation devices is given in figure 12.
Figure 12: M4A2 tank. Dead zones: driver and hull gunner - 5.3 m (slit), 4.4 m (periscope). Turret - 6.8 m (right), 31.8 m (left)
M3 Light
The driver and the hull gunner in the M3 Light tank have a mirror periscope each. There are two slits in the front of the turret platform. The slits are covered with thin glass and an armoured shutter.
Three mirror periscopes are installed in the turret. There are four slits in the commander's cupola. The slits are protected from the inside with glass. The placement of observation devices and range is given in figure 13.
Figure 13: M3 Light tank. Dead zones: driver - 3 m, gunner - 3 m, turret - 7.5 m, commander's cupola - 7.8 m (slit 1), 27 m (slit 2), 14 m (slit 3), 6.3 m (slit 4).
M5A1
The light M5A1 tank driver and hull gunner are placed in the front of the tank and observe through two mirror observation devices. The devices are located in the entrance hatches and can tilt and rotate.
The turret has a tilting observation device in the left side, but it cannot turn. There is the same device to the right of the gun, but it can turn. The placement of the observation devices and their range is given in figure 14.
Figure 14: M5A1 tank. Dead zones: driver - 5 m, turret - 6.8 m.
Conclusions
Old tanks have many observation devices. Improved vision was granted simply by increasing their number. As the hull design is simplified and new observation devices (periscopes) were used, the observation improved while the number of devices decreased.
In recent tanks, the driver has an observation slit and a mirror periscope. The hull gunner also has an observation device. The use of a movable periscope (tilting and rotating) is typical for Allied tanks. The British ones also have a mirror that allows them to look backwards. This increases the angles of observation.
Other crew members received similar periscopes. The observation devices of the latest tanks are reliable and provide good visibility. New Allied tanks do not have commander's cupolas.
I have some thoughts about tank vision devices. Firstly, at least some tanks use Polish Gundlach periscope (reversible periscope) on driver station, despite lack of usability of "reversiblity" on driver station (on driver station we can't use rear view capability of Gundlach periscope due tank turret which is located rear of driver station). Secondly- at least in some tanks, Gundlach periscope reversibility can't be used on commander station (late Sherman tanks with commander vision cupola, early T-54s). Thirdly- on late Sherman tanks I see strange off set between driver head and forward view periscope: https://milimoto.files.wordpress.com/2020/06/sherman-large-hatch-periscope-m.jpg
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