LIGHT. 121 



colour ; decoction of esculine a brown, &c. These were among 

 the substances first studied, but many others are now known to 

 possess the property. Fluorescence has been employed for study- 

 ing the invisible part of the spectrum beyond the violet. 



The remarkable experiments of Mr. Crookes on the repulsion, 

 connected with radiation, of light bodies suspended in a highly- 

 exhausted chamber have suggested a new method of measuring 

 radiation, or radiometry, as it is now generally termed. The main 

 facts of the experiments are as follows : If a beam of light falls 

 upon a light body suspended in a chamber which has undergone 

 only moderate exhaust! on,. flie body is attracted. This fact was 

 previously known, and it is attributed to convection currents. If 

 the exhaustion be carried still further, the attraction diminishes 

 and finally ceases. If the exhaustion be carried still further 

 repulsion ensues. To effect this degree of exhaustion Mr. Crookes 

 found it necessary to improve upon Sprengel's pump. The 

 amount of repulsion depends upon the colour, the substance, &c., 

 of the body suspended, and also upon the wave length of the rays 

 of light falling upon it. Various forms of these radiometers will 

 probably be exhibited. 



Next to polarisation, the most important branch of optics for 

 the study of the nature and properties of light is that of interference 

 and diffraction. This leads directly to the measurement of the 

 lengths of the waves of light. In what sense the term waves is 

 ultimately to be understood is a question which must some day be 

 answered by molecular physics ; but whatever be that answer, it 

 is at all events certain that the phenomena of light are periodic 

 both as regards time and space, and the wave theory of light is 

 undoubtedly the only theory which has as yet stood the test of 

 experiment. The fundamental experiment of producing inter- 

 ference by throwing together two rays of light by means of two 

 mirrors slightly inclined to one another forms a starting point in 

 this part of optics. As a complement to this we have the corre- 

 sponding experiment with the biprism. This is an instrument 



