LIGHT. 



I. OPTICAL INSTRUMENTS. 



THE instruments used in this branch of science may conveniently 

 be grouped under the following heads, viz., Production, Measure- 

 ment of Intensity and Velocity, Action of Matter on Light, Action 

 of Light on Light, Action of Light on Matter, Technical Appli- 

 cations of Optical Principles. 



The most obvious source of light is the sun j and for many 

 experiments ordinary daylight is sufficient; but when great in- 

 tensity is required direct solar beams reflected by suitable appli- 

 ances in the direction required are used. One great advantage of 

 light from this source is, that the rays are approximately parallel ; 

 while with light derived from other sources it is generally necessary 

 to render the rays parallel by a system of lenses. 



The instruments in general use for the production of light are 

 the lime-light and the electric-light. The lime-light is produced by 

 the combustion of mixed oxygen and hydrogen gases which play 

 upon a cylinder or cake of lime. Common coal gas is sometimes 

 substituted for hydrogen, but in that case the consumption of 

 oxygen is greater. The gases are sometimes contained in bags 

 placed under pressure, but it is now usual to compress them in 

 bottles of wrought iron. The jets used with hydrogen and with 

 coal gas are slightly different. Zirconia cylinders have been 

 suggested instead of lime; when these are thoroughly calcined 

 they answer well, but they have never become the subject of 

 general manufacture. 



