> -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Ethan Merritt > Sent: 12 June 2008 19:41 > To: Multiple recipients > Cc: CCP4BB@jiscmail.ac.uk > Subject: Re: [ccp4bb] birefringent spacegroups > > But the ellipsoid is only a convenient approximation based on > properties > evaluated at 3 orthogonal axes. It is not a complete > description, it is > simple model. More complex models may describe more complex > properties, > as is the case in the paper that Ian refers to. > > If "birefringence" is formally defined in terms of the approximating > ellipsoid, this is correct. I do not know if this is the case or not. > But if birefringence is generalized to mean "optical index varies with > incident illumination vector" then I believe all protein crystals > are birefringent. > > I may be talking nonsense here. If so, I welcome the opportunity to > learn better. > > Ethan >
No I think you're quite right, birefringence clearly refers to any case where the refractive index is anisotropic for any general pair of directions, not just in specific orthogonal directions. In any case the birefringent direction [110] in cubic is orthogonal to the non-birefringent direction [001] so the old definition is still applicable anyway. BTW here's a patent issued for a method to get around the problems in lithographic fabrication of semiconductors caused by optical elements made from birefringent cubic crystals, so it's a very real problem, not just one of theoretical interest! http://www.patentstorm.us/patents/6844972/fulltext.html Cheers -- Ian Disclaimer This communication is confidential and may contain privileged information intended solely for the named addressee(s). It may not be used or disclosed except for the purpose for which it has been sent. If you are not the intended recipient you must not review, use, disclose, copy, distribute or take any action in reliance upon it. If you have received this communication in error, please notify Astex Therapeutics Ltd by emailing [EMAIL PROTECTED] and destroy all copies of the message and any attached documents. Astex Therapeutics Ltd monitors, controls and protects all its messaging traffic in compliance with its corporate email policy. The Company accepts no liability or responsibility for any onward transmission or use of emails and attachments having left the Astex Therapeutics domain. Unless expressly stated, opinions in this message are those of the individual sender and not of Astex Therapeutics Ltd. The recipient should check this email and any attachments for the presence of computer viruses. Astex Therapeutics Ltd accepts no liability for damage caused by any virus transmitted by this email. E-mail is susceptible to data corruption, interception, unauthorized amendment, and tampering, Astex Therapeutics Ltd only send and receive e-mails on the basis that the Company is not liable for any such alteration or any consequences thereof. Astex Therapeutics Ltd., Registered in England at 436 Cambridge Science Park, Cambridge CB4 0QA under number 3751674
