OPTICAL INSTRUMENTS. in 



velocity of light may be calculated. These experiments have been 

 recently repeated, on a very large scale, by M. Cornu, at the 

 Observatory in Paris. 



For the verification of the gereral laws of reflection instruments 

 have been devised in which, the incident ray being fixed, the 

 reflecting surface is capable of being turned through any required 

 angle, and in which the corresponding angle of the reflected ray is 

 registered on a divided circle. Such, for instance, is the apparatus 

 of Jamin. Ordinary glass mirrors, when silvered at the back, give 

 rise to many reflected images ; the first, by one reflection at the 

 front surface of the glass ; the second, which is the brightest, by 

 one reflection at the silvered surface ; the third, by three internal 



reflections, and so on. To avoid this inconvenience metallic 

 mirrors have been used, and, latterly, glass mirrors silvered or 

 platinised on the front. Platinum is preferable to silver on 

 account of the rapidity with which the latter becomes tarnished. 

 But, owing to the great heat to which the glass must be subjected, 

 it is found very difficult to preserve the reflecting surface from 

 distortion. It is much to be regretted that, apparently owing to 

 these difficulties, the manufacture has been discontinued, both in 

 this country and in France. 



Convex spherical mirrors are used only for demonstrating optical 

 laws, and for decorative purposes. It has been suggested that 

 they still have another scientific signification, viz., that the images 

 produced may be regarded as diagrams appertaining to a non- 



