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According to the Online Etymology dictionary (http://www.etymonline.com/index.php?term=reflection):
reflection (n.) Look up reflection at Dictionary.com
late 14c., reflexion, in reference to surfaces throwing back light or heat, from Late Latin reflexionem (nominative reflexio) "a reflection," literally "a bending back," noun of action from past participle stem of Latin reflectere "to bend back, bend backwards, turn away," from re- "back" (see re-) + flectere "to bend" (see flexible). Of the mind, from 1670s. Meaning "remark made after turning back one's thought on some subject" is from 1640s. Spelling with -ct- recorded from late 14c., established 18c., by influence of the verb.

So it seems not to be an American/British spelling divergence in this case since both the original "reflexion" and the variant "reflection" have co-existed on this side of the pond since the late 14th c. (and I don't think there were any American English speakers around then!).  Both forms were in use in British English well before the European colonisation of N.America in the early 17th c. .

Also Wiktionary (http://en.wiktionary.org/wiki/reflexion) says:

reflexion (n.)
          From the Late Latin reflexionem, from reflexio; the variant spelling reflection is due to influence from correction.

Cheers

-- Ian


On 8 July 2014 22:53, Boaz Shaanan <[log in to unmask]> wrote:
I thought (I think I was told that way early during my PhD studies) that reflexion/reflection is a matter of British/American spelling. In fact Merriam-Webster Dictionary says just that:

Definition of REFLEXION

chiefly British variant of reflection 

 and the American Heritage and Oxford dictionaries agree on that too.

 Boaz

 
Boaz Shaanan, Ph.D.                                        
Dept. of Life Sciences                                     
Ben-Gurion University of the Negev                         
Beer-Sheva 84105                                           
Israel                                                     
                                                           
E-mail: [log in to unmask]
Phone: 972-8-647-2220  Skype: boaz.shaanan                 
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From: CCP4 bulletin board [[log in to unmask]] on behalf of Ian Tickle [[log in to unmask]]
Sent: Wednesday, July 09, 2014 12:19 AM
To: [log in to unmask]
Subject: Re: [ccp4bb] question about powder diffraction

Yes, the way I like to think of it as a double condition, the
reflection‐in‐a‐mirror condition *plus* the special condition imposed
by Bragg’s Law. This is why I often prefer the unfashionable spelling
“reflexion”.

--
Ian ◎

Me too.  Actually "reflexion" (but the verb is "reflect") is the original correct spelling (from Latin reflectere & reflexio); apparently at some point in its history it became misspelt due to a false analogy with "correct" & "correction" (Latin corrigere & correctio).

Now back to the science!  It's important to understand that a "powder" is not amorphous which would indeed give a continuous pattern: it's a bunch of micro-crystals in random orientations.  Therefore a powder diffraction pattern is a single crystal pattern averaged over all orientations.  Rotating the crystal does not change the Bragg angles of the spots, however it does change their angular positions so each diffracted beam is smeared out over conical surface.  Each of these cones then projects as a circle on a flat area detector (of course in powder diffraction one would use a linear detector since it's not necessary to measure a complete circle).

Cheers

-- Ian