Prof. Dove on the Dichrooscope. 355 



apparatus with feeble light the phenomenon is greatly attenuated, 

 and in ordinary daylight is not apparent, for the red rings then 

 quite disappear. Dichromatic combinations by superposing 

 different-coloured glasses are impossible with many colours, for 

 a pure red and a pure green glass become then quite opake. 

 These evils are obviated by the arrangement (c) . By combining 

 monochromatic and dichromatic glasses, which are placed at hf 

 and fg, any desired union of colours for polychromatic illumina- 

 tion may be obtained, which by covering either hf or fg are 

 immediately decomposed into their components. Indeed, by 

 combining a red and a green glass, Newton's rings with white 

 light at once appear. 



To make this more apparent, I have given in figs. 2, 3, 4, 

 5 the appearance of two crossed, plane, polished gypsum 

 crystals in red, yellow, green, and blue light, in which the 

 white lights in the figure must be considered as appearing in 

 the corresponding colour of the dark interference lines. If two 

 of these figures are imagined to be superposed, we obtain the 

 phenomenon of the combined illumination. As the square 

 which is common to both the gypsums shows five interference 

 lines in red and seven in blue, the alternation of red and blue 

 lines is then directly apparent, for the diagonals of both colours 

 appear black. A very instructive phenomenon is obtained, if, 

 while fg is covered by the slide, only one gypsum is examined 

 and at hf a glass is inserted, half of which allows one colour to 

 pass, and the other half the other colour. This sharp removal 

 of the interference lines is especially manifest. If it be desired 

 to show the part which each colour plays in the phenomenon 

 with white light, a glass is chosen for hf of which one half is 

 colourless and the other half coloured. 



I know no transparent bodies which transmit homogeneous 

 yellow light. Another arrangement was accordingly made for 

 this colour. The mirror c d was removed, the polarizing glass 

 substituted for the glass plate at ef and a condensing lens 

 placed at b. Near its focus was a spirit-lamp coloured yellow 

 by common salt, and immediately behind this a white flame ; 

 between these two there was a glass plate intended to colour 

 this flame. Since light of a different colour penetrates a homo- 

 geneous flame, the desired combination is attained in this 

 manner. 



In the place of well-polished gypsums, cooled glasses or com- 

 pensators of rock -crystal may be used. 



In order to extend the results obtained for interferences where 

 the same spaces are traversed with unequal velocities to inter- 

 ferences where different spaces are traversed with the same velo- 

 cities, the dioptric grating may be used. 



