This partial spectrum of a colour 840 C.F.L. was made using the "Pringles" can Spectroscope fitted with a D.V.D. as a monochromator in first blaze.
Attention is drawn to the yellow mercury doublet, at the left hand extreme of the group if yellow lines a and bands. These two lines are 2nm apart, 577 and 579nm. A dye on the D.V.D. blocks second blaze contamination, but the resolution of the mercury doublet is very good. One could "drive a truck" between the two lines!
The 437nm mercury line cannot be seen because it was obscured from the camera's view by a edge of the hole cut in the can.