According to the conventional wisdom, it was the Signal Corps until around 1870 when the Signal Service was formed from what was the Signal Corps. Or was it? Was it both during the war? How do we explain the large number of marine glasses bearing the markings "Signal Service" while none or very few bear the markings "Signal Corps?" We are pretty certain that there were few such glasses bought following the war and that most were turned in off of cleared form thirteens at the last encampment in June of 1865. So where are they? To add to the confusion and to support the claim that both SIGNAL CORPS and SIGNAL SERVICE were used to refer to the Union signal effort I refer you to the two following documents. Both are from early 1863 when Major Myer was "firmly" in control. Both are from the same book of General Orders. Notice also the other challenge to "conventional wisdom" in that it is clearly set forth that the heads of the various signal organizations within departments are to be Majors. Not just Myer. But this would not have been the case in 1862 if I read this document correctly. This was apparently new to 1863. In fact, the move was to create a Colonel of Signal with Majors at the Department level. Of course, Myer was not to benefit from this until after the war.
Note the reference to "signal service." What might the markings on issue glasses be? It would appear not everyone went along with Major Myer's grand view of a "Signal Corps."
Here the reference is to the "Signal Corps." The capitalization would lead one to think that this was the "official" name while signal service was not. Yet, there it is: both names used. I make no claims that "signal service" was official. I merely point out that it was in use in 1863 in General Orders.
History link showing the confusion around this time, before and beyond