and the Determination of White Light. 



219 



plane passing through the centre of the slit B of the mirror 

 H and the lens C. 



Light in passing through the prisms is bent through 

 about 98°. By the second lens it is brought to a focus at the 

 surface of the concave mirror, forming there a spectrum. 

 Thence it is thrown back, passing through the lower part of 

 the lens and prisms, whence, instead of being brought to a 

 focus at the slit, it is reflected by the plane mirror to the eye 

 of the observer. Light from the upper half of the slit passes 

 to the lower side of the spectrum on the concave mirror, and 

 is reflected to the microscopic slide of the plane mirror. 

 Similarly, light from the lower half of the slit passes to the 

 upper side of the spectrum, and the position of the slits deter- 

 mines the components that are allowed to be reflected. These 

 components, when reflected back through the system, make 

 up the mixture and strike the plane mirror. Each division of 

 the mirror gives an image from each part of the spectrum. 

 These images would be superposed upon a screen placed at 

 the focus U (fig. 1); but the mirrors are so tilted with 

 respect to each other that the upper field gives an image of 

 the mixture partially overlapping the image of the original 

 light from the lower Held. To an eye placed at the images 

 do not appear, but each part of the field is illuminated with 

 the light from the separate parts of the spectrum. 



A vanishing line is easily obtained between the two fields 

 by this arrangement, for a portion of the light is dissipated by 

 reflexion between the two silvered surfaces. The light is 

 slightly convergent, and the mirrors give a full illumination 

 up to the edge of the silvering. 



The wave-length corresponding to any position on the 

 millimetre-scale was determined by the same interference- 

 method that Maxwell used with his colour-box. In the first 

 determination 31 bands were visible. The following table 

 gives the wave-lengths corresponding to the different positions 

 on the millimetre-scale. 



