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1088 __ « Meng hsi pi-t'an' »''' (Dream Creek Essays) » — camera obscura
Shen Kua (1035-1095)
Comment : Talks of the camera obscura's inverted image, the collecting place, burning mirrors and the focal point. In his 'Meng hsi pi-t'an' (Dream Creek Essays), Shen Kua refers to the inversion of the shadow and goes on to say that when images are reflected in a burning-mirror (concave), they are inverted. He talks about the 'obstruction' as the place where the image disappears when reflected in the mirror. As he says, "It is also like the shadows of pagodas seen through the holes in windows" (Meng hsi pi-t'an (Dream Creek Essays), Passage on the Inversion of the Shadow, Shen Kua, 1088). Shen Kua presented his concept and understanding of the Mohist's 'collecting place'. He called it the 'obstruction' or an invisible place. He wrote how the image seen reflected from a concave lens will disappear between the center of the surface, and the focal point. This he called the obstruction. Something we now know to be the aperture where the rays of light cross and there is no image at that very point. This (right) illustration is taken from a summary abstract by Ch'en Yang in the year 1104. Kua attempted to show the waist-drum as an analogy of two shadows meeting, forming the invisible, or collecting place where it is the narrowest. The unlearned reader may understand this better with the observance of the following experiment; EXPERIMENT: Use a concave lens or mirror (curves inward). Place a small distinguishable object on the tip of a pin. Choose something that can be distinguished as being upside-down. Move the pin very close to the centre of the mirror and observe the object's image (depending on the size of the mirror, closeness of 1/2 to 3/4 of a centimeter may be required). Slowly pull the object back, keeping the image centrally aligned. Observe the image while pulling back. OBSERVANCE: Notice that while the image is close-up, it's reflection is seen in it's upright state. As the image is withdrawn from the surface slowly, there is a point in which the image is not seen on the surface of the lens. This then, is the 'obstruction', or collecting place that She Kua talked about. His oar in the rowlock so to say. As you pull the image back further, it appears inverted. CONCLUSION: In understanding this 'collecting place' more fully, consider the burning-glass effect. If the object is held indefinitely at the point of image-loss, and while being out-of-doors on a sunny hot day, the object likely will become flames or melt. This is because the heat of the sunlight being reflected and concentrated at this point is greatest. This phenomenon however is not restricted to light solely. It can be understood also in the use of micro, and sound waves as in the use of satellite and parabolic dishes. SEE MO TI. In explaining his understanding of this 1,400+ year old discovery of the Mohists, Kua used things in the sky such as clouds, birds and kites. He stated factually that if seen in the sky, the shadows of these objects naturally move in the same direction on the ground. However, when seen through an aperture such as the hole-in-a-window analogy, the object and it's shadow/reflection, go in opposite directions. Needham translated Shen Kua's Meng Chhi Pi. Than (on the inversion of the shadow) as; " ... The burning-mirror reflects objects so as to form inverted images. This is because there is a focal point in the middle (i.e. between the object and the mirror). The mathematicians call investigations about such things Ko Shu. It is like the pattern made by an oar moved by someone on a boat against a rowlock (as fulcrum). We can see it happening in the following example. When a bird flies in the air, it's shadow moves along the ground in the same direction. But if its image is collected (like a belt being tightened) through a small hole in a window, then the shadow moves in the direction opposite to that of the bird. The bird moves to the east while the shadow moves to the west, and vice versa. Take another example. The image of a pagoda, passing through the hole or small window, is inverted after being 'collected'. This is the same principle as the burning-mirror. Such a mirror has a concave surface, and reflects a finger to give an upright image if the object is very near, but if the finger moves farther and farther away it reaches a point where the image disappears and after that the image appears inverted. Thus the point where the image disappears is like the pinhole of the window. So also the oar is fixed at the rowlock somewhere at its middle part, constituting, when it is moved, a sort of 'waist' and the handle of the oar is always in the position inverse to the end (which is in the water). One can easily see (under the proper conditions) that when one moves one's hand upwards the image moves downwards, and vice versa. [Since the surface of the burning-mirror is concave, when it faces the sun it collects all the light and brings it to a point one or two inches away from the mirror's surface, as small as a hempseed. It is when things are at this point that they catch fire. This is indeed the place where the 'waist' is smallest.] ". (Compiled from various sources.)
Source : Wang, Ling and Needham, Joseph (1962), “Science and Civilization in China: Physics and Physical Technology : Physics”, Vol. IV, Cambridge University Press, pp. 97-99.
Urls : http://www.precinemahistory.net/900.htm (last visited )

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