It is possible for the eyepiece on your seawater refractometer to be removed for cleaning of any dust from the lens and can sometimes also become loose through constant screwing and unscrewing of the focus ring.
One main reason for the eyepiece to come off accidentally is due to over tightening the eyepiece down against the body of the refractometer when adjusting too vigorously or packing it away. Although this overzealous action can sometimes damage the plastic groove that the eye piece grub screws locate into, it is still possible to rectify this with relative ease.
1 - Remove the rubber cover from the eyepiece which at this point is probably detached from the unit if you are reading these instructions. It is now easy to see the grub screws that need to be tightened up with the screwdriver that is supplied with the refractometer.
2 - You will observe a black plastic part holding the loose lens which is sticking out of the body of the refractometer. Turn this in an anti-clockwise direction to screw it further out so that you can see the grub screw retaining groove and to make it easier to refit the eyepiece. Wipe any lubricant and finger prints away from the lens which you will see sits in a recess in the end.
3 – If the eyepiece is just loose and not completely removed then you can still pull and peel off the rubber eye piece / cup in the direction illustrated below and continue as above.
4 – Whether the eyepice is still on the refractometer or has alreay come off, the next step is to screw out the grub screws so that when you look from the inside there is bnothing seen to be prodruding. This will also allow you to remove a still fitted eyepiece.
5 – With the cap removed inspect the grub screw locating groove on the refractometer show in the image below. If the screw have been pulled over the channel and damaged the end make sure you clean loose plastic away and line the cap back up on assembly so they are secured in an undamaged part of the channel.
6 – Assemble in reverse order making sure that the grub screws are unscrewed all the way so they dont catch the channel when pushing the cap on. The cap should snap into place so you can tighten the screws equally on each side until they bite into the plastic groove and until you are sure that the cap is secure. The rubber cover piece is simply pushed back on.
This procedure should restore the refractometer to full working order but be mindful not to try to focus the eyepiece in or out so forcefully at the ends of its normal movement.