Divine Eye Development
Srila Yogeshwarananda proposes that because it will be near impossible for any student to consistently develop the divine eye while using a modern human body, one may been making progress towards the goal even without experiencing the divine eye. One may do on the basis of accepting the information from yoga-gurus that there is such an eye and that it will develop in time, provided one does the recommended practices.
One who has never seen lightning may be on the lookout for it after hearing about it from someone trustworthy who explains that one should look for it in the direction of a thunder clap during a violent rain storm. The likelihood is that there will be lightning but all the same it may not flash or one may not see it when it flashes because one looked in the wrong direction. Still, one may still look in the friction of the thunder clap during future rainstorms in the hope of seeing it.
If during meditation one ever sees light in the frontal part of the head and if that light is shimmering, glittering, dotty, spotty, flashing, or glaring. Then one should note it even if it lasts for moments only. One should note its spatial location. Then while meditating then or thereafter one should focus on that location even though there is nothing there.
This causes the development of stabilization of focus which is necessary for perceiving and using the divine eye. A mantra may be said while doing this focus where the light appeared, but the mantra should be one that is given or inspired by the same yoga-guru under who guidance one practices.
An example of how a mantra is bestowed is this:
Once when doing a focus on a space where light shimmers for some moments, and doing so after the shimmering ceased, Srila Yogesh appeared and instructed the use of the second mantra of Patanjali which is:
योगश्चित्तवृत्तिनिरोधः॥२॥
yogaḥ cittavṛtti nirodhaḥ
(Yoga Sutras 1.2)
Just based on his instruction, I used this mantra to reinforce the focus on that blank space where the light has appeared. But this does not mean that you should use the mantra. You may use it but it will not help you unless it was given by the yoga-guru. Even though I truthfully recorded this mantra and shared what happened, you should not confiscate this mantra and use it, because it will not be effective in that way.