observation of distant stations in geodcetical operations. 331 
the upright rod r fig. 3, and the flame of the spirit accord- 
ingly regulated so as to produce the greatest effect. A 
flexible tube leads from d to the vessel containing the oxygen, 
which may be either a common gas-holder, or perhaps a silk 
bag with a layer of caoutchouc, such as they are now made, 
might be conveniently employed for this purpose. The 
apparatus first made was provided with five jets, and could 
light up a ball inch in diameter ; that now represented has 
only three, and with it a ball £ of an inch in diameter may be 
used sufficiently large to admit of the requisite allowance 
being made for aberration in the reflector from its true 
figure, as well as uncertainty of direction arising from terres- 
trial refraction. 
To ascertain the relative intensities of the different incan- 
descent substances that might be employed, they were 
referred, by the method of shadows, to an argand lamp as a 
common standard ; the light from the brightest part of the 
flame being transmitted through apertures, equal in diameter 
to the small spheres of the different substances submitted to 
experiment. 
The result of several trials made at the commencement 
gave for 
Lime 37 times 
Zirconia* ... 31 times 
Magnesia ... 16 times the intensity of 
an argand burner. The oxide of zinc was also tried ; but 
besides wasting away rapidly, it proved inferior even to 
magnesia. 
• From the description given by Berzelius of the light emitted by zirconia 
before the common blow-pipe, a different result might have been expected. 
