Thanks Gustav and all for these interesting discussions; for the holothurians from Eparses Islands that I determined recently,  I used water as it is easy, but you are right it would be better to have a medium which allows the best conservation for future!

2014-07-22 15:37 GMT+02:00 Paulay,Gustav <[log in to unmask]>:

Hi All,


Thanks for all the suggestions.  I have looked a bit further, and although I do not fully understand the optics (but will find a microscopist to talk to…), the issue seems to be that calcite is birefringent – i.e. has two refractive indeces (RIs), at 1.4-1.6 and 1.5-1.9 (depending on light wave length), so how bright an ossicle looks depends on how these axes are oriented.  The problem is striking when you look at, for example, Holothuria with three dimensional buttons, where some buttons happen to lie on their “side” while others lie on their “back”.  Some buttons are then really well defined and others faint.  It is not apparent (though ossicles can look faint depending on their orientation) when viewing ossicles that all lie in the same orientation, as their optical axes then form the same angle with the viewing axis. 


The problem appears to be worse when IR of the mounting medium is close to that calcite, which is the case with basically all the mounting media we usually use, as manufacturers strive to have RIs match that of glass (1.52).  Refractive indeces of Euparal=1.48, DPX=1.52, Canada Balsam=1.54, Permount=1.54 all fall in this range, and although I have not tried them all, I suspect all have this problem.  Yves – this issue is clearly visible in your slides – e.g. see the fuscogilva type slides:


Other issues include the crystallization with age of Permount Dave suffered from (which supposedly has been fixed, but I remain vary), and the acidic nature of Canada Balsam.  I just checked some slides we made in CB a few years back and indeed the ossicles are dissolving.  Immersion oils also strive to have RI of glass, plus they do not provide a permanent mount.


RI of water is 1.33, so ossicles look much better in a simple wet preparation than in the permanent mount, a useful trick if you are producing photos for a pub.  However in the long run it would be ideal to have a mounting media that is archival and has an RI that makes ossicles nicely and uniformly visible.  I am not sure if a product like that is available. 


Cheers – Gustav



From: flmnh-aspidolist [mailto:[log in to unmask]] On Behalf Of Yves Samyn
Sent: Monday, July 21, 2014 2:48 PM
To: [log in to unmask]
Subject: Re: mounting medium


Hi Gustav


Euparal does the trick to me.


In an earlier life I used DPX to save histological preps; perhaps this works on ossicle preps as well ( Canada balsam seems fine too, but gives some decoloration...


cheers - yves

-----Original message-----
From: SCOTT SMILEY <[log in to unmask]>
Sent: Mon 21-07-2014 15:52
Subject: Re: mounting medium
To: [log in to unmask];

Dear Gustav

Cargil (I think it was Cargil) used to make a High Viscosity Oil that was fluid enough, when gently heated on a hot plate, to hold the coverslip down and did not crystallize. I do not know if it is still available.


On 7/21/14, 4:17 AM, Paulay,Gustav wrote:

Hi All,


I have been using Euparal for some time now as the mounting medium for cuke ossicles.  It does not have the crystallization – age problem of Permount.  However it has the wrong refraction index, so that some of the ossicles that have their optical axes in the “wrong” direction appear much fainter than others in these preps.  In contrast the same ossicles look terrific when viewed in just water.  Does anyone know of a better alternative – i.e. a permanent mounting medium without these optical issues that also does not crystallize?


Thanks – Gustav