436 ON A MONOCHROMATIC LAMP FOR MICROSCOPES, &C. 



would produce them in greater abundance than when it was in 

 a state of purity, and upon making the experiment, I found it 

 to succeed beyond my most sanguine expectations. The whole 

 of the flame, with the exception of a small portion of blue 

 light, was a fine homogeneous yellow, which, when analysed by 

 the prism, exhibited faint traces of green and blue, but not a 

 single ray of red or orange light. The green and blue rays which 

 accompanied the yellow flame, had comparatively so little in- 

 tensity, that they disappeared in the processes of illuminating 

 and magnifying the object under examination ; and, even if 

 they had existed in greater abundance, it was quite easy to 

 absorb them at once by the intervention of a plate of the palest 

 yellow glass, and thus render the lamp perfectly monochroma- 

 tic. 



From many experiments on the combustion of diluted alco- 

 hol, I found that the discharge of yellow light depended great- 

 ly on the nature of the wick, and on the rapidity with which 

 the fluid was converted into vapour. A piece of sponge, with a 

 number of projecting points, answered the purpose of a wick 

 better than any other substance, and the extrication of the yellow 

 light became more copious, by placing a common spirit-lamp 

 below the burner of the other. In order to obtain a very 

 strong light for occasional purposes, I connected with the top 

 of the burner a frame of wire-gauze, which, by moving vertical- 

 ly round a hinge, or by a motion to one side, could be placed 

 in a horizontal position about half an inch above the wick. 

 As soon as it had become red-hot, it was made to descend into 

 contact with the sponge, when it converted the alcohol rapidly 

 into vapour, and produced an abundant discharge of yellow 

 light. See Plate XXVII. Fig. 1. 



If a permanently strong light is required, I find it preferable 

 to dispense entirely with the use of the wick, and to allow the 

 diluted alcohol to descend slowly from the rim into the bot- 

 tom 



