LIGHT. 139 



halos will be seen by placing a small screen but a foot 

 or two from the glass. Fine rulings upon blackened 

 glass will in the same place give fine colors. These 

 rulings may be as coarse as fifty to the inch ; the finer 

 rulings will answer better. The rapidly-diverging rays 

 necessitate the placing of the screen close to the plate, 

 else the colors will be too faint. 



PERSISTENCE OF VISION. THE STROBOSCQPE. 



Let a disk a foot 

 in diameter be cut 

 out of any conve- 

 nient material, tin, 

 copper, zinc, o rj 

 pasteboard. Near 

 the periphery cut 

 out a number of 

 holes at equal dis- 

 tances apart, ten 

 or twelve will be I 

 enough. They may 

 be cut half an inch 

 is diameter. This disk is to be put upon a rotator like 

 the one used to show the Newton's disk, and may now 

 be placed so that the focus of the condenser with the 

 porte lumiere will be in the holes as the disk revolves, 

 as in Fig. 106. This permits the light to pass to the 

 screen only when the holes are at the focus, at which 

 time a powerful beam will pass and is immediately cut 

 off. With such a fixture a very great number of amus- 

 ing and instructive experiments may be made. 



i. While one person turns the stroboscopic disk 

 let another one Stand in front of the screen and swing 

 his arms, or move his body rapidly sideways, or make 



