110 A CHAPTER ON LIGHT AND COLOUR. 



sidered sufficient for the purpose, but Prof. Rowland of Baltimore 

 has devised means for ruling gratings which contain nearly 30,000 

 lines to the inch. Spectra of great purity are thus obtained, and 

 the results are highly satisfactory. 



19. — The colours of Mother-of-pearl and of Barton's 

 BUTTONS are due to the fine striation of the surface in either 

 case. Impressions of mother-of-pearl on black sealing wax shew 

 the brilliant tints almost as well as the original surface. Fine 

 powders, such as that of Lycopodium (the spores of Lycopodium 

 clavatum), when scattered on glass and viewed by transmitted 

 light, shew well-marked coloured rings, owing to diffraction, and 

 the rings seen round some stars, especially in small telescopes, are 

 due to the same cause. It is estimated that it would be impossible 

 to conceive of any microscope which would render visible objects 

 less than i/i 20,000th of an inch in diameter, owing to the inevit- 

 able diffraction effects connected with viewing them. 



20. — Colours of Thick and Thin Plates.— If the front surface 

 of a mirror be covered with a finely divided substance, such as is 

 obtained by brushing it with diluted milk, the light which is scat- 

 tered at the first surface, interfering with that which has undergone 

 regular refraction into and out of the glass and reflection at the 

 back of it, produces a very beautiful system of coloured bands 

 surrounding the image of the source of light. If the mirror be 

 concave, the source of light being at its centre, the rings appear 

 to have a very remarkable fixity of position, independent of the 

 position of the observer. 



21. — When a lens of small curvature is pressed against a plane 

 piece of glass, and viewed by transmitted or reflected light, there 

 is seen a system of brightly coloured rings surrounding the point 

 of contact. These are known as Newton's Rings. They are 

 produced by the interference of the light which emerges after 

 reflection, within the space between the lens and plate, with that 

 which is directly transmitted, or is reflected, at the first surface of 

 this space respectively in the two cases. The transmitted system 

 has a bright centre, the reflected system a dark centre. 



In the arrangement of lens and plate, the variations of thick- 

 ness of the interposed layer of air follows a known law, and hence 

 the wave-length of light of different colours may be calculated 



