of Electric Chronograph. 381 



may be taken as an example of the way in which the instrument 

 is used (see fig. 3) : — A piece of warm glass is smoked over 

 the flame of a paraffin lamp furnished with a wide wick ; it 

 is then attached to the carriage, and the stylus to be tested is 

 adjusted to the surface of the glass, also the tuning-fork is ad- 

 justed so that its writing-point lightly touches the glass ; an 

 electric circuit is then completed through the break I (fig. 2), 

 and the stylus prepared for giving a signal. The carriage is 

 then brought slowly past the stylus, the result of which is that a 

 vertical mark I H (fig. 3) is produced ; the carriage is then 

 held back by the detent, the stylus is again prepared, by the 

 armature being caused to touch the poles of the electromagnet, 

 the fork is excited, and the carriage released : the markings 

 KNML and A B are then produced. The tuning-fork point 

 is brought against the glass so that a straight line may be 

 drawn by it, the intersections of this with the curved line deter- 

 mine the limits of any vibration. The length of the traverse 

 I N, duly turned into time, is the latency of the stylus. Lines 

 are drawn by a needle's point through the points I and 1ST, cut- 

 ting A B in E F; the value of E F is then determined by means 

 of a micrometer microscope, constructed as shown in fig. 4. 

 In all cases of estimating time the fork is excited for each 

 observation, the writing-point of the fork being placed verti- 

 cally above the writing-points of the styli, so that the velocity 

 of the moving surface is common to both the fork and the 

 stylus. The needle-point for scribing the vertical lines is 

 carried on a kind of dividing-engine. The styli are found to 

 have a latency of almost perfect constancy, its value being 

 0*0003 second. The first stylus constructed by me, and used 

 in the Physiological Laboratory of the University, was not 

 proportioned as the later ones have been ; it was tested by 

 Prof. Gr. F. Yeo, and his result published in the 'Journal of 

 Physiology,' vol. ix. nos. 5 and 6, gave as a value of latency 

 0*00062 sec. The improved result, viz. 0*0003, has been 

 arrived at by a careful selection of the iron used, and a modi- 

 fication of the winding of the bobbins ; by the reduction of 

 the latency the marking is rendered much more definite and 

 readable. The two facsimile tracings (fig. 5) show the nature 



of the markings of a time-trace of a slow explosive wave. BA 

 or BA equals 0*00227". 



The micrometer-microscope is mounted on a bridge E F 

 (fig. 4) attached to an inclined table A B ; it is carried by means 

 of a slide, which permits movement parallel to the trace which is 

 put under it; the slide is moved by a screw having 40 threads 

 to the inch ; the screw has a micrometer-head divided into 25 

 large divisions, each of these is again divided into 4 parts, so 



