404 POPULAR LECTURES AND ADDRESSES. 



but there are optical difficulties in the way of 

 making, with satisfactory economy, groups of long 

 and short flashes, separated by short intervals of 

 darkness in the group, and comparatively long 

 intervals of darkness between successive groups ; 

 and considering how very much more useful and 

 satisfactory at sea is a lighthouse showing long 

 light with short intervals of darkness than even the 

 quickest of flashing lights, it does not seem desir- 

 able to push the distinctions of flashing lights 

 further than the double, triple, and quadruple 

 groups. The periods for these lights which seem 

 best adapted for practical purposes, all things con- 

 sidered, but most particularly their value to the 

 sailor, are as follows : 



It may be objected to the suggestions of the 



