122 SCIENTIFIC APPARATUS. 



consisting of a plate of glass, one side of which is plane and the 

 other is bounded by two planes, forming a very flat penthouse or 

 roof. In both these experiments the light, which really emanates 

 from one source, appears as if it emanated from two points very 

 near to one another. We thus have the confluence of two sets of 

 rays in the same phase of wave motion, which is an essential 

 condition in order that interference may take place. The biprism 

 is more convenient for use than the mirrors, but the calculation 

 of wave lengths is rather longer. 



Intimately connected with the foregoing is the subject of the 

 colours of thin plates, noticed first by Boyle, then by Hooke, and 

 afterwards in a much more complete manner by Newton. The 

 apparatus for these experiments, consisting of two glass plates 

 compressible by screws, is to be found in every laboratory. 



Interference of rays emanating from the same source is also 

 effected by diffraction, or the bending of luminous rays in their 

 passage near the edges of sharply denned objects. The pheno- 

 mena so produced are minute in dimensions, and are therefore not 

 generally adapted for projection. They are, however, extremely 

 beautiful when observed by the eye in a well-adjusted apparatus. 

 This consists mainly of screens, with openings of various forms 

 and sizes, supported on suitable stands with screws and other 

 appliances for position. These are usually fixed in grooves on a 

 strong iron frame called an optical bench. 



The diffraction figures, which are of course altogether different 

 in form from those of the apertures to which they are due, have 

 been the subject of much mathematical discussion. Bridge, to 

 whom some very general theorems are due, devised a very con- 

 venient form of apparatus, with apertures photographed on a glass 

 plate, by means of which a great number of beautiful figures may 

 be readily produced. 



There is a peculiar class of diffraction plates called gratings or 

 reseaux, which is of great importance. These consist of plates 

 of glass having a large number of very fine lines ruled upon them. 



