INTERFERENCE OF LIGHT 335 



magnificent gratings cut upon glass by Mr. Clarke, of nearly 

 3 inches diameter, which project brilliant spectral rings, 

 using a large focussing lens. A piece of coarse perforated 

 zinc held near the grating affords further beautiful pheno- 

 mena. 



Wire gauze can sometimes be procured fine enough to 

 give perceptible spectra, using a large lens and a piece 6 inches 

 square ; and spectra can often be obtained by diffracting through 

 a piece of the finest cambric, carefully selected, or through 

 the web of the feather of some fowls and birds. 



Circular halos are produced by diffracting a circular aper- 

 ture through a glass dusted with lycopodium powder, or any 

 similarly small spores ; on one condition. The ' resolution ' 

 of a grating depends upon the number of lines or dots brought 

 into use, and a dusted plate 3 inches diameter will not give 

 distinctly coloured halos on the screen. But plates 6 inches 

 diameter, used with a lens not less, and holding the plate 

 close to the lens, so as to pass the rays through the whole 

 effective surface, give really fine halos. These dusted plates 

 are best prepared by shaking the spores through fine muslin 

 from some little height, upon clean plates which have been 

 washed with water containing a few morsels of gelatine. If 

 gently breathed upon, the spores will be caused slightly to 

 adhere ; and after the surplus has been shaken off, a circular 

 mask of black card should be laid on the surface to preserve 

 from contact, another clean glass plate laid over this as a 

 cover, and the two bound all round with gummed slips of 

 dark paper, like lantern slides. Such plates should be of 

 thick glass, or the pressure of the fingers may bring the 

 plates into contact and spoil the even distribution of the 

 spores. Gratings and dusted plates can be mounted in 

 frames and used on stands if desired, but the hand is practi- 

 cally sufficient. 



192. Reflecting Gratings and Striated Surfaces. An 

 ordinary photographed glass grating will project very fair 



