As a matter of interest, let us realize the strong resemblance between the impression we receive from a material object actually seen by the eye and the memory of that impression afterwards. Let us then realize the strong resemblance between it and another impression of that same object seen mentally but not physically; for instance, let us realize the strong resemblance between the impression made on us by actually seeing some friend and the impression received by _imagining_ him receiving a letter which we are now writing to him. The first picture was an image of the external object that was physically made on the retina, as a picture or image is made by a camera on a screen; but that picture on the retina must have been seen by the brain, or we would not have known of it. The other pictures were not made physically on the retina, so far as we know. Yet we all realize that we can make pictures on our minds the more readily if we close our eyes. The fact of our eyes being open seems to operate adversely to our receiving a clear mental picture. Now it is a matter of fact that an object (for instance, a pole) can be seen by a person with normal eyesight, if it subtends an angle as great as one minute; that a pole a foot thick can be seen clearly from a distance of 3600 feet, at which distance it subtends that angle. The rays of light pass through the crystalline lens of the eye and are focussed on the retina, as they pass through the lens of the camera, and are focussed on the sensitized paper. Assuming the distance from the crystalline lens to the retina to be about three-quarters of an inch, the pole would be represented on the retina by an image 3/(4 x 3600) or less than 1/4000 of an inch wide. During daylight our retinas are continually receiving images of which all lines as wide as 1/4000 of an inch (and much narrower) are very clearly apprehended by the mind. But very few of those images are noticed by us. It is only when some incident calls them to our attention, or when the mind voluntarily seizes on them, that any conscious impression is made upon the brain. Similarly, images of physical objects unseen by the physical eye are continually made on the mind: we are continually thinking of our friends and of past incidents and possible future incidents; and our thoughts of these things take the form of pictures. We see the man with whom we had a conversation yesterday, and we see him with a clearness that is proportional to the interest taken by the mind in the conversation and the circumstances surrounding it. If our conversation was uninteresting and the circumstances tame, we see him dimly. But if our conversation was angry and the circumstances were exciting, we see him and the surroundings very vividly--so vividly that our anger is again aroused; perhaps to as high degree as on the day before, or even higher.