I am looking for the source and context of this quote, found e.g. at St Andrews:
It is onlyOnly with the greatest difficulty that one is one able to follow the writings of any author preceding Euler, because it was not yet known how to let the formulaeformulas speak for themselves. This art Euler was the first to teach.
— F. Rudio
(My emphasis. Slight variants suggest that it could originally have been in another language.)