AND INSTRUMENTS AT MUNICH. 509 
the copper plate and to substitute one of glass. After this change 
the instrument continued as perfectly free from oscillation as 
before, so much so that in observation each reading gave the 
position, and it became unnecessary to eliminate the oscillations 
by repeated readings. 
One condition was still required for exact reading with this 
instrument. The small size of the mirror did not permit the use 
of the ordinary scales (on white lacquered paper) with fine di- 
visions. I prepared scales of glass, which instead of reflecting 
the light received from the front by the illuminating mirror, 
transmit the light of a mirror placed behind. Besides simplicity 
of construction this mode of illumination has the advantage of 
affording much more light, and rendering it possible to apply to 
small telescopes higher magnifying powers than they would other- 
wise bear. A telescope of 7 lines aperture suffices for reading 
a scale of which the divisions are equivalent to only 10". I used 
scales in which the single divisions had an angular value of 30". 
I had thus a new declination instrument fitted for the most 
delicate researches. I soon noticed the frequent occurrence of 
strikingly rapid changes, consisting in a repeated progression 
and retrogression in very short intervals, which however were 
at least four times as great as the time of vibration of the needle 
itself. I had therefore a second instrument made of the same size, 
with the view of satisfying myself whether these movements 
were really produced by the magnetic force or by extraneous 
influences (such as oscillations of the air, for example). The 
result was, that the two instruments agreed exactly in these 
_ small movements, proving thereby not only that small needles 
are fit for the investigation of the magnetic force, but also that 
they possess a double advantage over larger bars, first in being 
free from vibration, and secondly in showing variations which 
eannot be shown by a bar having a considerable moment of 
inertia. 
I did not however consider that I had yet done enough to justify 
the use of small magnets, particularly as the examination of the 
relations of the force in question to surrounding substances and 
localities has been by no means thoroughly exhausted. There 
is but one satisfactory proof in regard to magnetic instruments, 
namely, that different instruments should give, under all circum- 
stances, accordant and therefore strictly comparable results. 
