A CHAPTER ON LIGHT AND COLOUR. 103 



4. — If a source of light, such as a candle flame, be placed 

 behind a pinhole in a card, an image of the light giving body may 

 be obtained by interposing a screen in the path of the light which 

 passes through the hole. The image is found to be inverted, and 

 if a sensitised plate be substituted for the screen, a fairly well 

 developed photograph may be obtained of any object placed in 

 front of the pinhole. Other holes will give other images, and by 

 the continued multiplication of holes the individuality of the 

 images may be destroyed and a tolerably uniform illumination of 

 the screen, within the shadow of what remains of the card, results, 

 due to the overlapping of eacli image by its immediate neighbours 

 on all sides. Unless the source of light be quite small, the shadow 

 of an opaque object or the light transmitted through a hole will 

 not be terminated by a sharp edge. 



The light gradually becomes fainter as we approach those 

 regions which are so placed as to receive light from a part only of 

 a luminous body. This partially bright border is called the 

 Penumbra, and its consideration is of great importance in the 

 calculation of the circumstances of eclipses. 



5. — The fact that light is not instantaneously transmitted from 

 point to point was proved by Roemer in 1676 from a consideration 

 of the eclipses of Jupiter's moons, which appear to take place 

 earlier or later than predicted, according as the earth is at the 

 part of its orbit nearest to, or furthest from, Jupiter. The range 

 of eight minutes on either side of the average in the time at which 

 these eclipses are seen is due to the extra distance, equal to the 

 diameter of the earth's orbit, which the light has to traverse in the 

 one case compared with the other. 



In 1725 Bradley showed that a small displacement of the appa- 

 rent positions of stars was due to the fact that the velocity of light 

 is only about 10,000 times that of the earth in its orbit. The 

 Aberration is a phenomenon exactly analogous to the apparent 

 slope of rain drops, really falling vertically, as seen from a swiftly 

 moving train. From Bradley's observations, and a knowledge of the 

 sun's distance otherwise gained, the velocity of propagation of 

 light could be determined. 



6. — This determination, however, was made directly by Fizeau 

 in 1849 t>y the use of a rapidly revolving toothed wheel. It was 



