This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Next to the burners, the shape of the globes or shades surrounding the flame affects the illuminating power of the light. In order to obtain the best results, the flow of air to the flame must be steady and uniform. Where the supply is insufficient the flame is likely to smoke; on the other hand, too strong a current of air causes the light to flicker and become dim through cooling.
Globes with too small openings at the bottom should not be used. Four inches should be the smallest size of opening for an ordinary burner. All glass globes absorb more or less light, the loss varying from 10 per cent for clear glass to 60 per cent or more for colored or painted globes. Clear glass is therefore much more economical, although where softness of light is especially desired the use of opal globes is made necessary.