174 On the Production of Monochromatic Light, 



of the lens. Thus the greater the deviation required the nearer 

 to the margin of the lens the part of it employed should be. 

 In other words, the cylindrical lens acts as a series of prisms 

 of varying angles. To obtain mixtures of colours to corre- 

 spond to the deviated patch, movable slits, and capable of being 

 narrowed or widened, are placed on each side of the fixed slit. 

 By this plan two patches of light of equal size and equal inten- 

 sity can be readily produced. When measurements are to be 

 obtained, scales are attached to the various slits, by which any 

 part of the spectrum can be identified ; and the widths of the 

 slits are measured by a gauge. 



Since this apparatus was described I have referred to a 

 paper by Helmholtz, which appeared in PoggendorfF's An- 

 nalen in 1855, in which one of the methods he used for the 

 combination of coloured light to produce white light is de- 

 scribed. The general principle he adopted is the same as that 

 described above ; but in several important details the latter 

 differs considerably from Helmholtz's apparatus. For instance, 

 the apparatus now described is suitable for the comparison of 

 colour-mixtures with monochromatic light of any colour and 

 for their exhibition on the screen for lecture-purposes, and an 

 illumination is secured which is very largely in excess of that 

 usually obtained. In a paper read before the Physical Society 

 (Phil Mag. June 1885) Lord Eayleigh shows how a mono- 

 chromatic image of an external object may be seen by placing 

 a concave lens immediately behind the slit of the spectroscope 

 of such a power as to throw an image of that object on the 

 prism. I have found that by altering the distances apart of 

 the collimating lenses and viewing lens, a monochromatic 

 image of the sun may be thrown on the screen. If such an 

 image be coloured with the light of the blue or violet hydrogen 

 lines, it should be possible to photograph the solar prominences 

 en bloc. I may mention that the apparatus as described was 

 employed in two Cantor Lectures at the Society of Arts in the 

 beginning of April, but without describing it in detail. 



I am at present engaged in using this apparatus for inves- 

 tigating some phenomena existing in colour-blindness, and 

 obtaining curves of illumination of different lights, and also 

 in some photographic researches. These results are not yet 

 ripe for publication ; but I have thought it might be well to 

 publish the method employed, as it is one of great conve- 

 nience, and very easily carried out by any one who has a 

 spectroscope and a photographic camera. 





