Why Mirror Images Appear Deep: Lucretius' Explanation in "On the Nature of Things"

In part IV Lucretius' poem On the Nature of Things, at line 269, Lucretius introduces his new topic of images. He claims that everything on earth is constantly emitting tiny particles of colour in an outward direction which make up their appearances when combined. For example, if one were to combine the particles emitting from oneself together we would have a replica of ourselves. These tiny particles pass through many objects, but when they hit mirrors, they are forced to rebound back and be perceived by our eyes which is why we can see ourselves and other objects in mirrors.

Lucretius then goes on to explain why images in mirrors appear to be deep and receding "into its depths" (271). He claims that two currents of air are the cause of this phenomena.

Similar to an Opened Door:

The mirror image, as Lucretius says, is similar to when we perceive things "in their reality" (272) through an opened door which allows us to have a perfect view of the things outside the house from the inside. In this case as well as in the case of the mirror images, the vision is caused by two currents of air.

First, the air between ones self and the doorposts is perceived, followed by "the leaves of the door itself on either side" (276), then the light beyond the door reaches and passes through our eyes which is followed by a second current of air, then the objects outside "that we see in their reality" (278).

In the case of the mirror, when it is turned in the direction of our eyes, it pushes forward and drives the air between ourselves and the mirror causing us to perceive this air first. We then perceive the mirror itself. This is followed by the image of ourselves being thrust forward in the direction of the mirror. The image stream is stopped from going any further by the mirror and is then reflected back to our eyes. However, before we perceive the image of ourselves, we perceive another current of air which has been driven forward by the returning image of ourselves. This is why the "reflection appears to be so far behind the mirror" (286f). The two currents of air cause us to perceive depth. This is why both the mirror image and the items beyond the door appear to have depth and be farther away.

After perceiving the doorposts themselves, the second current of air reaching our eyes before the images of what lies beyond the door reaches our eyes and causes us to perceive the images beyond the door as farther away. Similarly, the same happens with the mirror. Once perceiving the mirror, another current of air passes through our eyes before the rebound image. This causes the rebound image to appear to be further and receding into the mirror.

Explaining Reversed Images

After explaining how double currents of air cause us to perceive depth, Lucretius then explains why we see the left side of our body in the right side of the mirror and vice versa. This happens because when the image of our body reaches the mirror and "strikes" (294) against it, it does not turn around unaltered, it rather rebounds straight back. Lucretius uses the example of a plaster mask being hit against a post. What will happen is that the mask will "mold a copy of itself in reverse" (300). The face of the mask will now be facing in the opposite direction or be looking toward where the back of the mask used to be. The result is that the previous right eye will now be the left eye and vice versa.

The mirror image encounters a similar event when what was previously the left side of the body is now the right side. The image thus experiences a turn when it strikes the mirror. This turn is inevitable because if we were to look at a mirror and if the image darting toward the mirror were not turned, we would only experience a rear view of ourselves rather than an image that is facing us. Because this turn is necessary, we then experience the previous left side of the body as the right and vice versa. For it is true that when a person stands facing us, the left side of their body is actually on the right side of our body.

Source:

All references are to line numbers in part IV of Lucretius' poem, On the Nature of Things, trans. Martin Ferguson Smith, Indianapolis IN, Hackett, 2001