The exercise is meant to obtain Central Fixation (Trataka) at a near point.
Method: Look at the black dot from a distance of 72 cms, and observe its darkness with both the eyes. After gazing at the dot for some time you will feel it has got darker and a flame is seen moving like in a fire.Now close your eyes. You will see the dot in your imagination and your mind will feel relaxed.
Figure 42: Imagination Exercise - I
Here is another variation of the Central Fixation exercise, which you can try in the same manner.