Is stacking welder's glasses a safe way to watch at the eclipse? You can find in many place on the Internet that welder's glass #14 is good for looking at an eclipse. Tomorrow (March, 20th 2015 at 10:45 CET) there's a solar eclipse and yesterday I could only find glasses #11 and #9. I tried briefly this morning, and combining a #11 and a #9 was giving me a good view of the sun. Using 2 #11 gave a too dark image. Now, I read different opinions on the internet.

Against (Perkins Observatory):
http://perkins.owu.edu/solar_viewing_safety.htm


  Be careful that you use the right kind of glass! Welder's glass is
  numbered from 1 to 14 with 14 being the darkest. It is only number 14
  glass that is dark enough for solar viewing! And NO STACKING! A pair
  of number 7's or a 10 and a 4 together DO NOT have the same protection
  as a single piece of number 14 (see unsafe methods for more details).


Favorable (Royal Astronomical Society of Canada):
https://www.rasc.ca/tov/safety


  If SN14 filter is not available, it is possible to combine lower shade
  numbers to get roughly the same level of eye protection from solar
  radiation, e.g. combining SN 6 and SN 8 filters. However the image
  quality may be considerably poorer than that seen through the single
  SN14 filter


I could not find a table or something explaining which kind of protection gives each number; according to the Canadian website, the only concern is about how much infrared light goes through, ultraviolet does not seem to be a problem in almost any case (I was surprised to read that).

Note: It's not my intention to open here the discussion on what could be other safe methods to watch the eclipse, this is well explained everywhere around. I read too late about the eclipse to order specific glasses.

Edit:
One of the answers here report the following formula: (more insight at this link)


  13 or darker is safe enough. Also, you CAN add up welding glass, using
  the formula S(sum) = S1 + S2 -1. S(sum) should be greater than or
  equal to 13
YES

You have summarized pretty much everything available on the subject. There are loads of people saying you cannot, but fortunately for us, the Royal Astronomical Society of Canada, that you quote, includes evidence to back up their claims. They are a respected institution and the author of this article appears immensely qualified. The article itself does not include evidence that welding goggles can be stacked, but the inclusion of such for all the major points adds credibility to the entire article.

It does appear like the glasses are not exactly additive, this Reddit post goes through some calculations, but they are pretty close such that aiming for 1 tint higher will more than offset it and if anything is overkill.