"No man will keep Christ before his mind without having to make a sensible effort to turn away from the whole rabble of distracting thoughts that lie round him. In Hebrews the writer lays it down as a condition of all persevering continuance in the race set before us that we should be "looking to Jesus"; and he employs there a word which might be rendered, perhaps, "looking away" to Him. That conveys the same idea of rigid shutting out of other things in order that one supreme light may fill the eye and gladden the soul. If you do not carefully drop black curtains round the little chamber, and exclude all side lights, as well as all other objects from the field of vision, there will be no clear impression of the beloved face made upon the sensitive plate. It must be in the darkness that the image is transferred to the heart." Alexander Maclaren. D.D.
I like this illustration of a dark room being the place we transfer the image to the heart. It is pregnant with applications, and I was going to write what he said after this but decided to stop here, it leaves it open to interpretation.
Photo from internet