On the Production of Monochromatic Light, 173 



on to the screen belonging to a camera, B. This screen was 

 placed at an angle with the axis of the lens L 3 as shown, so that 

 a fair focus of every visible ray was obtained upon it. (It may 

 be worth mentioning that a hair placed across the slit or a 

 little particle of dust is a good means of obtaining a focus 

 when Fraunhofer or bright lines are not observable. The 

 black streak produced by it should be sharp along the whole 

 of the spectrum.) A card, D D, with a slit S 2 or slits cut in 

 it, replaced the ordinary dark slide, and, by moving it along 

 the spectrum, any colour or colours can be allowed to pass. 

 Before using the apparatus the whole of the spectrum was caused 

 to fall on a convex lens, L 4 , of about 24 inches focal length and 

 about 5 inches diameter. This collected the dispersed beam of 

 light, giving an enlarged image, F, of one surface of one of 

 the prisms on a screen, E. By placing this lens at an angle 

 with the axis of the lens L 3 , the blue and red fringes can be 

 made to disappear almost entirely, and a practically w r hite 

 patch of light is seen on the screen E. 



I may say that the lenses used are white flint of medium 

 density and almost colourless, even in great thickness. 



When the adjustments are complete, as the slit is moved 

 along the spectrum every patch of colour or colours will suc- 

 cessively occupy the same position on the screen and have 

 the same area very nearly. We thus can have patches of 

 monochromatic light of any colour or combinations of any 

 colours, all other colour being absent. 



I have also obtained the same results by substituting mir- 

 rors for all the lenses and a reflecting-grating for the prisms; 

 but I do not see any particular advantage in this plan, as the 

 white light is more tinged (with the colour of the metal) 

 than when prisms and lenses of white glass are employed. 



When the source of light is the arc light, if an image of the 

 crater of the positive pole be thrown on the slit S x of the 

 collimator, the intensity of the light is such (when the slit 

 is fairly open) that the patch of nearly pure light may be 

 made 1 foot square, and yet be sufficiently brilliant to be seen 

 by a fairly large audience, and for an ordinary lecture-room 

 it is very effective. The mixture of colours to imitate any 

 colour in the spectrum may be shown by placing a narrow 

 slit in a small card in the colour required to be imitated, 

 and fixing in front of it and in contact with it a portion 

 of a cylindrical lens, the axis of the cylinder being of course 

 parallel to the slit. This throws the image of the particular 

 colour to be observed at one side of the image that would be 

 obtained were the cylindrical lens absent, and any amount of 

 deviation can be given the patch by using the different parts 



