Let's start with a relatively modern source:
The right time at which to introduce the hops, and the method of so doing, depend to a great extent upon the construction of the copper and the class of beer brewed. Some brewers subject the total quantity of hops to the entire period of boiling. Others reserve about a quarter of the amount to be added to the copper about 10 minutes before the boiling is completed. The latter idea is, in our opinion, the better, as it appears to impart to the wort some of the delicate flavour of the hops which prolonged boiling destroys. In the case of an open copper, the hops may be added with little trouble. With a pressure copper it is almost impossible to add them at this later stage, as it would entail letting down the pressure and opening the dome. As an alternative, we advise reservation of 1/8 of the total quantity to be sprinkled on the plates of the false bottom of the hop-back. This procedure will be found to give the desired effect. The method is especially suitable for pale ales when a delicate hop aroma and flavour is so necessary. There is no need to follow this process in the case of mild ales, or black beers, when a predominant hop flavour is not an outstanding feature."
"Brewing a Book of Reference", 1947, pages 114-115.
Just two additions are recommended: at the start of the boil and 10 minutes before the end. That's the latest hopping mentioned so far. Until the alternative method suggested, that is. That one's after the end of the boil. Though notice that it's only recommended for Pale Ales. Sounds rather like modern IPA brewing techniques.
Moving back in time a couple of decades,here's another view:
Make up the copper within four hours from first setting taps. It is imperative to fix the worts constituents by heat as soon as practicable in order to prevent undesirable changes in the underback or copper before boiling.
If worts are boiled in two successive coppers, or lengths, a proportion of the hops should be saved for the second copper, and the spent hops of the first copper returned."
"Practical Brewing and the Management of British Beers", W.H. Nithsdale and A.J. Manton, 1924, pages 36-37.
That's quite a long boil being recommended: two to two and a half hours. Whitbread rarely boiled for longer than 90 minutes in the 1920's. Barclay Perkins boiled a little longer, but boils of more than 2 hours were usually only for higher-gravity beers. The exact opposite of what's stated in the quote.
The text is a bit vague about hop additions. It's two additions again. I assume the first is at the start of the boil (though it isn't explicitly stated. The last addition is, again, 30 minutes before the end of the boil.
The last paragraph - if I understand it correctly - recommends re-using the hops from the first copper in the second. Exactly what W.E. Wright said you shouldn't do. Not very consistent, are they, these old sources?
There's still lot's more to come in this series. I'm just having too much fun to stop.