of Magnetical and Meteorological Instruments. 331 
Every part of the apparatus from which light can be reflected, 
directly or indirectly, to the paper, is carefully blackened. 
have described the apparatus at some length, because with 
the exception of the speculum, the cylinder itself, the time piece, 
and the tube of the barometer, all of which must, at present, prob- 
ably, be procured from Europe, there is nothing which an ingen- 
ious mechanic might not execute at a small cost ; the fixed lenses 
should perhaps be added ; but a simple substitute for them may be 
found iu a well blown cylinder of thin glass filled with alcohol, 
as was indeed employed by Mr. Brooke in his first instrament. 
The more expensive leuses at present in use are manufactured by 
Lerebours of Paris, and may be described as lenticular prisms 
having a double convex section, but forming a flat bar, which is 
from seven to eight inches long and one inch and a quarter wide 
for the magnetical curve, and about five inches long for that of the 
barometer and base line. Each lens is mounted on a light frame 
which slides in a groove so as to admit of adjustment of focus. 
The focus is rather improved by covering a portion of the margin 
of the lens. 
It is advisable on the first adjustment to bring the centre of the 
mirror, of the cylinder, and of the slit before the lamp, into the 
same horizontal plane, and to establish some marks for its recov- 
ery in successive adjustments, as much of the perfection of the 
focus depends upon this circumstance. If it is found that the 
mirror when thus adjusted, throws the image of the slit too high, 
or too low, its own inclination must be altered by introducing a 
small wedge of cork behind it. Small derangements produced 
Y spontaneous alterations in the length of the suspension silk 
may be corrected by raising or lowering the lamp, and must be 
carefully attended to. 
tion of iodid of potassium employed, the object of which is, to in- 
sure the permanence of the effect for the long period of twenty-four 
hours, during which a part of it, at least, is to be maintained upon 
