382 



On a New Form of Electric Chronograph 



that 4L x T Jp 



40V of an inch can be read with ease. The 



microscope is furnished with a fine fibre which is brought 

 over the trace to be measured; a rod of rectangular section 

 H K is attached to two links or rods, LH, M K; the links 

 being equal and moving about the points M, cause the rod to 

 move always parallel to itself; upon the rod the trace rests; 

 any part of the trace can be brought under the microscope. 

 The markings appear as rather wide lines of light; a V-shaped 

 scale in the field of the instrument enables one to bisect these 

 lines. Fig. 3 gives an illustration of its use. The centre of 

 the field is brought over C, the index of the micrometer-screw 

 being at zero ; then the microscope is moved by the screw till 

 the centre coincides with D. The length C D is then re- 

 corded, let it be L ; then the micrometer is brought to zero 

 again, and E is brought under the centre, and E F is then 

 measured in the same manner as C D, let it be I ; then, if 

 x denote the time of traverse over E F, and t the time of one 

 vibration, 



L : I : : t : x, 

 and 



- l l 



x- L - 



Fractions of a vibration can thus be easily estimated with 

 great accuracy. This proportional method of subdividing the 

 vibrations depends upon the fact that the velocity of the 

 carriage is practically uniform. 



The method of preserving time-traces. — After the smoked 

 glass has received records of time-traces it may be preserved 

 by being varnished. It is a somewhat difficult matter to 

 cover a smoked plate with photographic varnish without 

 making either streaks on the plate or removing a good deal of 

 the carbon surface ; but if the varnish be diluted with about 

 25 times its volume of strong methylated spirit, it may be 

 poured over the plate without injuring the trace. When it 

 has dried off ordinary photo-varnish may be applied without 

 any risk of doing harm. In the first case the plate should 

 be cold, in the second it may be a little heated before a fire 

 previous to the application of the thicker varnish. The traces 

 can then be used as negatives to print from in the usual way. 

 Eastman's bromide-paper in rolls has been found to be most 

 convenient for reproducing the traces. One of these is shown 

 at fig. 5. The lines are thicker than those usually made, as 

 a thicker deposit of carbon was used to get a very dense 

 negative to make a print for this communication to the Philo- 

 sophical Magazine. 



