Mr. T. Smith on certain Pheenomena of Vision. 343 



bring the bubble of the additional level by means of its pinion 

 exactly to between the two marks drawn across its tube*. 

 Continue thus successively to depress the telescope by the 

 pinion L until its bubble stands at the reversing point, and 

 afterwards to elevate it by the pinion J until the bubble of the 

 additional level comes to between its marks ; and when the 

 repetition has been carried far enough, the altitude may be 

 found by dividing the mean of the two readings by the num- 

 ber of observations. 



Apparently, there would be no difficulty in obtaining any 

 multiple of the double zenith distance of a celestial object pro- 

 cured precisely as by the French circle. Two additional 

 levels, both fixed to the divided circle, would then be indis- 

 pensable; one to be levelled when the instrument had been 

 turned half round in azimudi, and the other when the tele- 

 scope had been pointed the second time at the star. 



By means of the two additional levels, the difference of 

 zenith distance of two objects might be measured at once, 

 without obtaining the absolute zenith distance of either, 



Leeds, Sept. 4, 1832. J. NixON. 



Erratum.— Page 108, line 1, for 11" read 11', 



LXIV. Livestigation of certain remarkable and unexplained 

 Phcenomena of Vision, in 'which they are traced to Functional 

 Actions of the Brain. By Mr. Thomas Smith, Surgeon, 

 Fochabers. 



[Concluded from p. 258.] 



pjAVING thus ascertained the precise nature of the effects, 

 -*- -* the next step I took was to investigate the true nature of 

 the exciting cause. In all the experiments in which I had hi- 

 therto observed the phenomena, the light which appeared to 

 excite them was more or less white; for the bright object was 

 either a lamp or candle, or the direct rays of the sun, or the 

 sun's light reflected from snow or the like. In addition to this 

 circumstance, the bright object had always been so situated in 

 relation to the point P, to which the eyes were accommodated, 

 that its image on the retina must have been formed either 



* It would be preferable to have both levels fitted up with accurate 

 scaler, and to note the position of their bubbles rather than to attempt to 

 bring them always to one fixed mark. From tlie register of the deviations 

 of tlie bubble of the telescope from its reversing point, and those of the 

 bubble of the additional level from the point at whicii it stood when the 

 telescope pointed at the star, it can be ascertainedjiow many seconds are 

 to be added to or subtracted from the final readini'. 



