70 THE TRANSATLANTIC LONGITUDE. 



certainly exists in its value for the same observer, when different instruments are 

 employed for its measurement ; the magnifying povi^er, and the amount of light, 

 each appearing to exert a distinct effect. 



The second is a subject of considerable interest ; and extended series of com- 

 parisons between the observations of the same persons, using eye-pieces of different 

 magnifying power with the same instrument, and using instruments of different 

 aperture with similar reticules and eye-pieces, could not fail to afford much informa- 

 tion. It had long been my desire to carry out this investigation, toward which, 

 indeed, a considerable amount of materials has been collected, but for the present, 

 at least, no facilities are within my reach. It is certain that persons of the most 

 delicate nervous organizations are not generally those who observe a transit earliest; 

 nor does the reverse hold true. And it would seem that an influence is here in- 

 volved, which does not exist in the method of observation by eye and ear; viz., an 

 (generally unconscious) effort of judgment, by which many, if not most, observers 

 give their signal-tap, not at the instant when the star is seen upon the thread, but 

 at such a previous moment that the signal may in their estimation take effect at 

 the instant Avhich it is desired to record, after the lapse of an interval of volition 

 and an interval of muscular contraction. It is readily seen that if an observer 

 succeed in attaining this end for both equatorial and circumpolar stars, it can only 

 be by a very accurate estimate of the apparent rate of motion of the star, and that 

 a change of eye-piece for the same star will produce an effect analogous to a change 

 of declination in the star observed. The true method to be aimed at, in chrono- 

 graphic observation, clearly is to give the signal at that instant when the star is 

 actually seen to be bisected. Then, however large the personal difference from 

 other observers, the personal equation becomes constant, unaffected by many ex- 

 traneous influences, which cannot otherwise fail to exert a perturbing influence. 

 Still, the attainment of this end is by no means entirely within the observer's con- 

 trol, but must, under any ordinary circumstances, vary with the organization and 

 training of the individual. The strictly psychophysical part of the personal equa- 

 tion, is, as I have already remarked, merged with such other parts as depend upon 

 the observer's habitude. Yet it is clear that all these portions are in general not 

 constant, but vary to a great extent with the position of the star, and probably 

 with other external circumstances. It is probably in this element, also, that the 

 well-known variation takes place according to tlie condition of the observer. 



The third element, the key used, is generally of more importance in those chro- 

 nographs on which the signals are given by the closing, or making, of a circuit, than 

 on our own, all of which are arranged for break-circuit signals,- inasmuch as in the 

 former case it is usually needful for the contact-piece to be moved through an 

 appreciable space before tlie signal is given, while in the last-named arrangement 

 the first motion of the contact-piece breaks the galvanic circuit, and records itself 

 upon the chronograph. But if the spring, which maintains the contact when the 

 button is not pressed, be stronger than usual, or not nicely adjusted, there is danger 

 that an observer accustomed to the use of a more delicate key, upon which a touch 

 suffices to produce an interruption of the circuit, may either fail to record his sig- 

 nals at all, or in default of this may consume an appreciable time in exerting suffi- 



