i2 4 SCIENTIFIC APPARATUS. 



formed by gratings are called normal spectra on account of the 

 absence of the unequal dispersion due to prisms in which only 

 one ray can pass at the angle of minimum dispersion. They 

 have been much, used in determining wave lengths, as by 

 Angstrom, Thalen, and others. 



The effects of light so far noticed, with the exception of photo- 

 graphy, are transient; but some bodies have the property of 

 retaining for a short time and of re-emitting the light rays to which 

 they have been subjected. This is called phosphorescence. A 

 great number of substances have been found to possess this pro- 

 perty, although in many cases the duration is extremely short, in 

 fact a very small fraction of a second. M. Becquerel, who has 

 made the subject a special subject of study, has devised a phos- 

 phoroscope for measuring these short durations. This phospho- 

 rescence t is apparently connected with fluorescence, and accord- 

 ingly the electric light, which is rich in violet rays, is well adapted 

 for the purpose. Some compounds of sulphur with calcium, 

 barium, &c., giving very beautiful results, are now prepared. 



WM. SPOTTISWOODE. 



