Hi Rita,
I've found the reference, it was in the Complete Encyclopedia of Needlework by Therese de Dillmont a free copy here
http://www.gutenberg.org/ebooks/20776. (bother, it doesn't show the illustrations if you read it online).
Found a better source!
http://encyclopediaofneedlework.com/index.htm
This copy has different chapter headings to mine, ignore the chapter called Irish Lace but go to
http://encyclopediaofneedlework.com/chapter_4.html#fig_139
Chapter 16, Practical Directions also has some interesting information, about halfway down it deals with laundering of lace.
http://encyclopediaofneedlework.com/chapter_16.html
If you want to go ahead with trying to mend the collar, you will in all probability need a magnifying glass, lots of time & lots of patience (I would also suggest practising the technique on a separate piece of net that you have made a hole in). You will need a pad (6-8 layers of calico tacked together?) to pin/tack the collar on to, to support it and keep it in shape whilst working it and I would recommend that you have some plain light/mid blue glazed chintz (furnishing fabric) directly under the lace, as it will give you a nice smooth surface to slide the needle along & make it easier to see the lace. For threads you would need to track down some lacemakers. I can send you a sample of some fine cotton thread that I have, but you would need to identify whether the collar is made from cotton or linen. Apart from the thread thickness, you could also have the problem of thread colour - how many different shades of white/cream can you find?
These are some of the the questions/solutions that I can come up with after a nights pondering.
For an easier but more ££ option I would recommend contacting Jacqueline Hyman at
http://www.textilerestoration.co.uk/ I went on a weekend course she ran on ecclesiastical textiles, it was excellent and she is really nice.