266 PHYSICS. 



vibrated tnning-fork was used to regulate and measure the speed, the 

 light from the revolving mirror being reflected from a steel mirror on 

 one of the prongs to a piece of plane glass in front of the eye-piece, and 

 thence to the eye. When the mirror makes as many turns as the fork 

 makes vibrations, or when these are in simple ratio to each other, the 

 two images become stationary. The electric fork made 128 vibrations, 

 and was compared at each set of observations with a standard Uts fork 

 making 256.072 vibrations at 65° F. The deflection was measured with 

 an accurate micrometer. The results are given in 100 readings, each of 

 which is the mean of ten separate observations. The mean value cor- 

 rected for temperature, for the velocity of light in air is 200,804 kilome- 

 ters per second. Adding 80 kilometers to reduce it to vacuo, this value 

 becomes 299,9-14 kilometers, with a probable error of ±51 kilometers. — 

 This gives 180,380 miles per second reduced to English measure. — {Am. 

 J. Scl, III, xviii, 390, l^ovember, 1879.) 



Ayrton has given a lecture at the Eoyal Institution upon magic mir- 

 rors, the effects of which, as it appears, so far from being intended, were 

 quite unknown to the Japanese, though known to the Chinese from the 

 earliest times. On reflecting a divergent beam of light from such a 

 mirror, Ayrton found that the pattern on the back appeared bright on 

 a dark ground ; with a parallel beam no pattern could be seen ; while 

 with a convergent beam the pattern was dark on a light ground. Hence, 

 the eflect must be caused by inequalities in tlie curvature of the reflect- 

 ing surface corresponding to the raised pattern on the back, the portions 

 where the relief increases the thickness of the plate being flatter than 

 the remaining convex surface, and sometimes actually concave. The 

 result is very exceptional, being shown well in only 2 or 3 i)er cent, of 

 the mirrors made. It is produced when the bronze casting is worked 

 sufiiciently thin to determine a buckling of the metal. — {Xafure, xix, 186, 

 539, December, 1878; April, 1879.) 



Govi presented to the Turin Academy in 1864 a valuable historical 

 paper upon the magic mirrors of the Chinese, called by them Tlicon- 

 Icoiiany-kim^ or mirrors which the light can penetrate. In this he shows 

 that if the mirror be heated from behind the thin portions heat most 

 rapidly, become more convex, and the magic eilect is made more marked. 

 Bertin, associating himself with Duboscq, sought to improve upon this 

 suggestion of Govt's, by varying the curvature by ])ressure. A frame 

 was constructed for holding the mirror so that air could be forced in 

 behind it to make its surface more convex, or exhausted to make it con- 

 cave. A mirror in nickel-plated brass, on the back of which figures 

 were engraved and relief ornaments soldered, became very magic by 

 pressure, giving simultaneously a dark image of the depressed designs 

 and light images of those in relief — {Ann. Chim. Phys., V. xx, 99, 110, 

 143, May, 1880; J. Phys., ix, 401, December, 1880.) 



Bibart has shown that although the images produced in the kaleid- 

 oscope are sharp when looked at directly, whatever be the angle between 



