LIGHT. 113 



In connection with the subject of reflection mention should be 

 made of instruments designed to send a beam of solar light in one 

 definite direction at all hours of the day, by means of clock-work. 

 Among these the best known are Foucault's and Silbermann's 

 Heliostats. A somewhat simpler and less expensive instrument 

 has been devised by Professor Stoney. But the most perfect of 

 all is that known as the Siderostat, described under the head of 

 Astronomy. 



It would perhaps be going too far into detail to enter upon the 

 subject of quadrants, sextants, reflecting circles, &c., used for 

 the astronomical and geographical determination of latitudes, 

 and for surveying. 



Refraction, or the bending which a ray of light undergoes in 

 passing from one medium to another, varies, as is well known, 

 with the media used in the experiment, with the angle of inci- 

 dence, and also with the colour of the light. Instruments on the 

 same principle as those used for reflection are employed for deter- 

 mining refraction. The quantity which is the subject of measure- 

 ment, is the ratio of the sines of the angles of inclination of the 

 incident and refracted rays respectively, to the normal of the 

 bounding surface of the two media at the point of incidence ; or 

 it is, what is the same thing, the ratio of the velocities of light in 

 the two media. The principal instrument in aid of this question, 

 as well as many of those which follow, is the prism, whereby the 

 rays composing white light are variously refracted or dispersed. 

 This can be used for the question of refraction if we employ 

 monochromatic light such as that produced by sodium or by 

 spirits of wine and salt. Or more generally, it can be so used 

 if we employ as a source of light a spark producing a bright 

 line spectrum, and take account of one bright line only at a 

 time. Or again, it can be so used, if we employ solar light and 

 register the results obtained in respect of only one dark line at a 

 time. 



If the substance be solid and transparent, it can be cut into a 



