S. P. Langley — Observation of Sudden Phenomena. 99 



scope, b b is the inner tube, resting on friction wheels ff, 

 revolved by the clock-work c c about once a second, and 

 recording the time at which a key in the observer's hand may 

 be pressed to indicate the particular second. This record may 

 be made electrically by the wires w w on a chronograph, or 

 more simply and directly on a little attached dial like that of a 

 recording stop-watch. 



pp is the prism of double total reflection, r r is the position 

 of the fixed reticule (shown independently as it appears to the 

 observer and of full size in figure 2). 



e e are the lenses of a positive eye-piece. The lamp is for 

 giving wire illumination, if desired, when a telescope is em- 

 ployed. 



Field illumination is readily obtained otherwise by making 

 the diaphragm in which the prism pp is set of translucent 

 material. 



Finally it should be remarked that on removing the eye- 

 piece events may be observed without using any telescope. In 

 this, its simplest form, the chronograph may also be dispensed 

 with, and the record of the second made on an attached stop- 

 watch dial and the instrument may thus carry its own complete 

 recording apparatus and be more portable than an ordinary 

 opera-glass. 



I have not found opportunity to use this apparatus on the 

 moon or occultations, but I have, what is possibly more to the 

 purpose now, tried it on an artificial star, the instant of whose 

 appearance and disappearance was independently recorded on a 

 chronograph by an electrical contact. Different observers, 

 entirely unskilled and ignorant in the use of the instrument, 

 were invited to look into it and to determine the quadrant and 

 section in which the star appeared and disappeared. 



I have momentarily mislaid my notes containing in full 

 detail the results of four observers but I can summarize them 

 approximately by saying that after being simply told what to 

 note ; the average probable error (that is, for any single 

 observation) was rather less than one-twentieth of a second. As 

 far as I can judge from the limited number of instances, the 

 younger the observer the better the observation. The worst of 

 the observers (the oldest), however, had a probable error con- 

 siderably less than one-tenth of a second ; the youngest, a 

 probable error of something like one-fortieth of a second, 

 which implies, as you will observe, that he not only readily 

 noted the quadrant and the subdivisions of the quadrant, but, 

 also as a rule, even the part of the subdivision in which the 

 star was first seen. None of these observers had so much as 

 one hour's practice. 



