290 MR J. CLERK MAXWELL ON COLOUR, 



(3.) Union of Coloured Beams by a Prism so as to form one Beam. 



The mode of viewing the beam of light directly, without first throwing it on 

 a screen, wasnot much used by the older experimenters, but it possesses the ad- 

 vantage of saving much light, and admits of examining the rays before they have 

 been stopped in any way. In Newton's 11th proposition of the 2d Book, an experi- 

 ment is described, in which a beam is analysed by a prism, concentrated by a 

 lens, and recombined by another prism, so as to form a beam of white light simi- 

 lar to the incident beam. By stopping the coloured rays at the lens, any pro- 

 posed combination may be made to pass into the emergent beam, where it may be 

 received directly by the eye, or on a screen, at pleasure. 



The experiments of Helmholtz on the colours of the spectrum were made 

 with the ordinary apparatus for directly viewing the pure spectrum, two oblique 

 slits crossing one another being employed to admit the light instead of one vertical 

 slit. Two pure spectra were then seen crossing each other, and so exhibiting at 

 once a large number of combinations. The proportions of these combinations 

 were altered by varying the inclination of the slits to the plane of refraction, and 

 in this way a number of very remarkable results were obtained, — for which see 

 his Memoir, before referred to. 



In experiments of the same kind made by myself in August 1852 (indepen- 

 dently of M. Helmholtz), I used a combination of three moveable vertical slits to 

 admit the light, instead of two cross slits, and observed the compound ray through 

 a slit made in a screen on which the pure spectrum is formed. In this way a 

 considerable field of view was filled with the mixed light, and might be compared 

 with another part of the field illuminated by light proceeding from a second sys- 

 tem of slits, placed below the first set. The general character of the results agreed 

 with those of M. Helmholtz. The chief difficulties seemed to arise from the de- 

 fects of the opticala pparatus of my own eye, which rendered apparent the 

 compound nature of the light, by analysing it as a prism or an ordinary lens 

 would do, whenever the lights mixed differed much in refrangibility. 



(4.) Union of two beams by means of a tra .sparent surface, which reflects the first and 



transmits the second. 



The simplest experiment of this kind is described by M. Helmholtz. He 

 places two coloured wafers on a table, and then, taking a piece of transparent 

 glass, he places it between them, so that the reflected image of one apparently 

 coincides with the other as seen through the glass. The colours are thus mixed, 

 and, by varying the angle of reflection, the relative intensities of the reflected and 

 transmitted beams may be varied at pleasure. 



In an instrument constructed by myself for photometrical purposes two re- 

 flecting plates were used. They were placed in a square tube, so as to polarize 



