ILLUMINATION. 
79 
and the writer prefers a flat wick to a round one, though 
some excellent work has been done with the round 
wick. The lamp recommended is made by Swift, and 
is shown in fig. 25. 
About an ounce of camphor added to a pint of the 
best paraffin will produce a light more than usually 
white and actinic. The slip of glass in front of the 
flame is apt to break, especially when the flame is 
turned edge to the microscope as we recommend. To 
prevent this mishap the slip should be boiled for an 
hour in water or, better, in oil, the slip being wrapped in 
a cloth during the boiling. 
The lime—or oxyhydrogen—light is more costly to 
inaugurate, but we find it the most satisfactory of all 
the illuminants. It can always be relied upon ; it is 
very equable if we take some simple steps to make it 
so. The light is strong and chemically active, and we 
can use the “ critical ” image (the radiant actually 
focussed on the object) without projecting unsightly 
markings on the negative. 
Two kinds of jets are in use : the “ blow-through,” or, 
as it is sometimes called, the “ safety ” jet, and the 
“ mixing ” jet. In the former, coal gas from the house 
main is led through a comparatively wide tube to the 
nozzle, where it burns at the ordinary pressure ; a 
stream of oxygen is “ blown-through ” the burning coal 
gas, and the two gases impinge together on a piece of 
limestone, or a piece of material partly composed of 
lime, and known as a “ soft lime.” The mixing jet is 
about three to four times as powerful as the blow- 
