74 



Lord Haijleigli 



[March 24, 



plate of glass, the intervening space being filled with oil of turpentine 

 or other fluid having nearly the same refraction as glass. Moreover, 

 the light should be purified from blue rays by a trough containing 

 solution of bichromate of potash. With these precautions the dark 

 parts of the bands are very black, and the exposure may be prolonged 

 much beyond what would otherwise be admissible. 



The lantern slides exhibited showed the elliptical rings indicative 

 of a curvature of the same sign in both directions, the hyperbolic 

 bands corresponding to a saddle-shaped surface, and the approximately 

 parallel system due to the juxtaposition of two telescopic " flats," 

 kindly lent by Mr. Common. On other plates were seen grooves due 



Fig. 2. 



to rubbing with rouge along a defined track, and depressions, some 

 of considerable regularity, obtained by the action of diluted hydro- 

 fluoric acid, which was allowed to stand for some minutes as a drop 

 upon the surface of the glass. 



By this method it is easy to compare one flat with another, and 

 thus, if the first be known to be free from error, to determine the 

 errors of the second. But how are we to obtain and verify a 

 standard ? The plan usually followed is to bring three surfaces into 

 comparison. The fact that two surfaces can be made to fit another 

 in all azimuths proves that they are spherical and of equal curvatures, 

 but one convex and the other concave, the case of perfect flatness not 

 being excluded. If A and B fit another, and also A and C, it follows 

 that B and C must be similar. Hence, if B and C also fit one 



