Mechanical Science. Ml 



netism is always in the direction of the length, whilst with peroxide 

 of iron, wood, or gum lac, it is mostly in the direction of the width, 

 and always when only one magnetic bar is employed, whatever 

 may be the direction taken by the substance. 



Tliese differences of effect, which establish a line of demarcation 

 between the two sets of phenomena, are dependent upon the cir- 

 cumstance that the magnetism being very small in the latter bodies, 

 the reaction of the substance upon itself maybe neglected, and con- 

 sequently the action of the magnet upon the substance is the most 

 powerful agency. — Annates de Chimit; xxxvi. 348. 



10. On the apparent Motion of a small Body in the immediate 

 Neighbourhood of one larger or more brilliant upon which the Eye 

 remains fixed. — The star ^ of the Great Bear is accompanied by 

 one much smaller marked g in Flamsted's catalogue. Upon look- 

 ing attentively at these two stars, I thought I saw the smaller 

 agitated, moving irregularly near the greater, approaching to it, 

 receding from it, and again moving to the right or left. On making 

 other persons observe the stars, I found I was not the only one 

 susceptible of the illusion ; I have frequently repeated the experi- 

 ment since, on these and other stars. I applied this effect to ex- 

 plain the irradiation or enlargement of the diameters of the stars, 

 which, though it has usually been considered a physical effect, may 

 very probably belong to the theory of sensation. 



I was desirous of finding some effect analogous to that which I 

 had observed, and to which I could have recourse, when reflecting 

 upon the subject ; and one clear morning, when in bed, I thought I 

 saw an iasect upon the window which moved in different directions 

 and rather slowly upon the glass ; but upon rising, I found that the 

 imaginary insect was a black spot fixed' upon the pane. 



I then made a black spot three or four lines in diameter upon a 

 white wall, and about a line from it anofhermiich smaller. Remov- 

 ing to a distance of from fifteen to eighteen f^Qi, into a situation 

 from which I could view them attentively, I saw the small spot 

 agitated near the' large bhie" exactly like the star g-, near the ^. 

 This experiment proves that the motion is not diie to the cause to 

 which the phenomenon of scintillation is usually referred. I at first 

 thought it might be occasioned by a very sUght involuntary and 

 unperceived convulsive riiotibn in the ball of the eye ; but as the 

 whole of the ball would then move, the respective situation of the 

 two spots should not be changed by that cause. 



Perhaps this phenomenon is related immediately to our method 

 of perception, and therefore is unsusceptible of explanation. It is, 

 however, a fact, which has only to be well observed and clearly 

 described, and may then be used to explain other similar efft?cts. 

 It is possible that it is caused by a motion of the nervous pulp of 

 the retina itself; and indeed, the nervous or cerebral pulp of an 

 animal has been seen to move in the manner supposed. It ap- 

 pears not unlikely that the whole nervous system of a living animal 



