A CHAPTER ON LIGHT AND COLOUR. 109 



ness and darkness, some of the former exceeding in intensity the 

 uniform illumination outside the shadow. There are also bands 

 outside the geometrical shadow. As before, these are more con- 

 spicuous in monochromatic than in white light, and the scale being 

 different for different colours, the overlapping of the colours rapidly 

 destroys the clearness of the bands as seen by white Hght. 



17. — Similar Bands of Colour may be observed within the 

 shadow of a narrow object such as a hair, or around the border of 

 the light falling through a small hole on a screen. A very strange 

 effect noticeable in the case of the shadow of a small circular 

 object is that, at determinate distances from the source of light, 

 the centre of the shadow is a bright spot, the colour of which 

 varies as the screen is moved towards or from the obstacle which 

 obstructs the light. 



Even more strange is the effect produced when light falls 

 through a small circular aperture. At one particular distance of 

 the aperture from the source of light and the screen, the illumina- 

 tion at the centre of the bright spot is four times as great as if the 

 screen which contains the aperture were altogether removed, and 

 by doubling the size of the aperture, instead of increasing the 

 illumination, we obtain a black spot at the centre of the illumina- 

 ted space. These effects of interference are due to exactly the 

 same causes as in the case of sound, but the extreme smallness 

 of the wave-length of light, which may be taken to be about 

 i/5o,oooth of an inch for light of mean refrangibility, compared 

 with ordinary objects which produce shadows, and with ordinary 

 apertures, render them inconspicuous in general. 



18. — The bands of colour obtained in Diffraction Phenom- 

 ena, when white light is employed, and before the mixing of 

 colours has masked all variation of intensity of light, may be made 

 to afford a spectrum of great purity, and the dark lines and bands 

 of the refraction spectrum are very readily studied in the diffrac- 

 tion spectrum when^ by due precautions, it has been made to 

 extend sufficiently in length. The measurements of wave-length 

 are more readily made in the spectrum as produced by this 

 method. For these purposes, the light is generally reflected from 

 a surface on which a series of equidistant parallel lines are ruled 

 very close together. Formerly, 3,000 lines to the inch was con- 



