1 34 Mrs. M. Somerville on the magnetizing power 
they were free from magnetism. The needles were placed 
in various directions in the plane of the magnetic meridian, 
sometimes in the angle of the dip, sometimes perpendicular 
to the magnetic meridian, and also at various angles with 
regard to it. In some cases the heads of the needles were 
exposed in place of the points, to the violet rays. Perhaps it 
might have been expected that the influence would have 
been greater in those instances in which the needles were 
placed in the plane of the magnetic meridian, and at the 
angle of the dip ; and, consequently, polarity might have 
been expected to take place in a shorter time under these 
circumstances ; yet in fact there seemed to be no difference ; 
most of the needles became magnetic, some in longer, others 
in shorter periods, varying from about half an hour to four 
hours, but depending on circumstances which I have not yet 
been able to detect, further than that a number of results 
induced me to believe, that the experiments were more suc- 
cessful from ten to twelve, or one o'clock, than later in the 
day. The portion of the needle exposed was almost always 
a north pole, whether it pointed upwards or downwards. In 
a few instances in which the contrary occurred, it may possi- 
bly have arisen from some previous disposition in the needle 
to magnetism, too slight to be observed. 
The distance of the needle from the prism was frequently 
varied by fixing the needle to the wooden pole of a fire- 
screen, but without material variation in the effect. I found 
it unnecessary to darken the room ; it was sufficient to place 
the prism so as throw the spectrum on any place out of the 
sun's rays. 
My next object was to endeavour to ascertain whether any 
