This listing is for an illuminator unit that fits into sub-stage mount where the mirror is normally located in Watson Bactil & Patna model microscopes - from around 1930s through to late 1960s. It may also fit some Service models.  The unit fits into a recess in the base of the lower limb and in the listing photos I was fitting it to a vintage Watson Service from the 1920s and in this age of Service the illuminator only fits when the instrument is inclined.  Once the fitting is inserted, it can be tightened via a chromed thumb-screw as shown and when installed it's held in position well.  The bulb shown has not been tested, although will probably work OK - it looks intact.

In order to fully use this component a 6V power source will be required (this is not supplied), therefore this listing is for the plug-in illuminator unit only as shown in the listing photos.  The connector cable currently has a 3-pin fitting as shown.

I've also included a page in the listing from a 1950s Watson brochure that shows this type of unit and its application.  If there's any questions re fit or suitability for your Watson, please just contact seller.

The unit will be well wrapped and shipped by Royal Mail.

Thanks for looking.