28 
PRACTICAL PHOTO-MICROGRAPHY 
found to work best at about one or other of the 
standard lengths named, and the apochromatics are 
made deliberately for a short or for a long tube, and do 
not work well except at about the tube length for which 
they are intended. On the question—which is prefer¬ 
able, the short or the long tube ? the writer has no 
opinion to give. The one is as good as the other, only 
we find, and others find, that a long tube apoc hroma tic 
works fairly on a sho rt tu be, but, as a rule, a short tube 
one does not perform well on a long tube. A tube may 
safely be made adaptable from about .6 to 8 inches^or 
from about g to 11 inches ; any greater draw is apt to 
be weak. 
