Unusual microscope objective

[old martParticipant @oldmart]

Trawling ebay as usual, I chanced upon an objective which doesn’t make sense to me.

It is a Cooke 8X 0.2 oil. I have a 40 X, 90X and several 100X oil, but only 8X?

The vendor cannot get it to focus, and the ebay number is 186906405232.

[SillyOldDufferModerator @sillyoldduffer]

Bump!  For the attention of MichaelG who I think explained in another forum.  Won’t steal his glory, better from the horse’s mouth.

Low magnification oil immersion objectives feel wrong but there’s a reason, new to me!

Focussing would require one to be very close to the microscope slide.  Is the slide end spring loaded such that the objective has to press on the cover slip to get the lens inside into position?  Protective sheath gummed up with old oil maybe, enough to break the slip.

Dave

[Michael GilliganParticipant @michaelgilligan61133]

The most likely usage known to me … but there may be more …

It can be very convenient to have a lower power objective for ‘field scanning’ to identify a region of interest: ergo, if the ‘main’ objective is already an oil-immersion one, it makes sense for the low power to also be oil immersion.

Leitz and Nikon both favoured supplementary caps, but there is no reason to suppose that Cooke could not make the feature integral.

What this does not explain, however, is why the Seller has failed to focus it !

… I suspect that it’s just a matter of “must try harder”

MichaelG.

.

Edit: __ this might be the other discussion which Dave had in mind:

https://www.model-engineer.co.uk/forums/topic/unusual-cooke-microscope-objective/

… if nothing else, it gives some indication of Cooke’s position at the leading-edge of microscopy.

[Michael GilliganParticipant @michaelgilligan61133]

Perhaps a little esoteric for this forum … but this discussion on ‘microbehunter’ might be of some interest:

https://www.microbehunter.com/microscopy-forum/viewtopic.php?t=13173

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

For anyone mildly curious about Oil Immersion … this is a predictably useful primer from Cargille:

https://cargille.com/wp-content/uploads/2018/03/Immersion_Oil_and_the_Microscope.pdf

MichaelG.

[old martParticipant @oldmart]

Typical of me not to twig the obvious reason for a low power oil immersion objective. The pictures shows a grub screw in a slot and some corrosion around a possible sprung nose which may be jammed in the retracted position. The seller might be reluctant to try such a low power objective as close as a 100 X would be used.

The other Cooke that I came by,the 45X LWD oil in the link to my earlier thread was the opposite, the oil drop had to be big to hold together when used.

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