The left hand image is a schematic of the Crookes' tube arrangement.
The tube had gas at low pressure inside it and electrons which original from the cold cathode were accelerated towards the anode.
A lot of the electrons were travelling so fast that they were unable to turn the corner towards the anode and there went onward towards the Maltese cross along almost straight line trajectories.
Some hit the Maltese cross but a number reached the glass and caused fluorescence and the relatively sharp image of the Maltese cross.
What is interesting is that the electrons which hit the Maltese Cross leak away and do not affect other electrons coming towards the Maltese Cross.
The voltages used were very high but even so the flux of electrons was not.
A modern tube uses an electron gun and has a screen with a phosphor on it so much lower accelerating voltages can be used.
The Maltese cross is also at the same potential as the final anode of the electron gun to try an ensure that the trajectories of the electrons are relative straight to produce a sharp shadow.
An interesting effect can be obtained by removing the high potential lead from the Maltese Cross and this is shown on the right hand image.
Because of the high electron beam currents significant numbers of electrons are stored on the Maltese cross which the repel the electron beam.
The sharp shadow of the cross being produced by visible light emitted from the electron gun.