November 22, 1607
How to Make a Comet
Here is a method of representing a reflection very like a comet:
Take a clean carafe and hold a lighted candle not far from it, and you will see in its surface a tiny image of the light, very sharp and bright.Next with the tip of your finger take a small quantity of any oily material that will adhere to the glass, and spread a thin coating where the image appears, dimming the surface a little.The image will be promptly dimmed too.Now turn the carafe so that the image emerges from the oiled spot and just touches its edge, and rub your finger once right across the oiled part.Instantly you will see a ray formed in imitation of the tail of the comet, cutting right across the place where you rubbed your finger.If you rub across this again, the ray will be led off in another direction.This happens because the skin on the ball of your finger is not smooth, but is marked with certain twisted lines which we use in sensing the slightest irregularity of objects by touch.These leave some tracks in moving over the oily surface, and the reflection of light takes place in their edges, and since they are numerous and regularly arranged this forms a light stripe.The image may be placed at the head of this stripe by moving the carafe, and will then appear brighter than the tail.The same effect may be produced by fogging the glass with the breath instead of using oil.
I do not mean to imply that there is in the sky a huge carafe, and someone oiling it with a finger, thus forming a comet. I merely offer this as an example of Nature’s bounty and variety of methods for producing her effects.I could offer many, and doubtless there are still others that we cannot imagine. I shall write more about the subject of effects very soon, in a parable which I am certain you will enjoy. GG