398 
Transactions of the Society. 
The image of an artificial star is focussed by means of the latter, 
and the objective is inclined so that the light passes through it at about 
twenty to thirty degrees from its axis. If the ocular be then turned 
slightly out of focus, the objective, unless properly corrected, presents 
in the eye-piece a series of coloured riDgs quite apart from any want 
of chromatic correction. 
In the case of a lens under-corrected in this sense, the outer rings 
are red on both sides of the focus ; but if the air-space already referred 
to be carefully adjusted while the objective is under examination, these 
rings can be made to coalesce exactly, so that they present a ring of 
white light. 
Finally, I wish to express my deep obligation to Messrs. Taylor, 
Taylor and Hobson for many interesting details concerning these 
lenses, and the loan of two of them for testing and experimental 
purposes. 
