Hi , I'm James DVM MRCVS and would like to help you with your problem
sorry to hear about Jasper.
By the sound of it he is not feling very well
This Marble effect which you were referring to is usually due to a corneal damage of some sort (like an ulcer or a scratch for ex) which can lead to bacterial infection . That is something which needs medial attention as soon as possible , since infection of the cornea can go real bad real quick without proper antibiotics
It could be that his hardship of getting don the stairs , his lack of balance is originating in the same problem or it might be something else all together .
I would advise strongly to go an see you vet this morning for him or her to take a look at the eye and examine Jasper all together to see if his incidental vomiting and lethargy is related to the injured eye .
It would recommend to run a very basic blood test . one that would asses his general well being and give us quick and reliable information if anything is going wrong .
Meanwhile do no try and clean or wash the eye with water .
as it can make things worse .
Make sure he is well hydrated (in case he is not drinking , that is even more of a reason to visit your vet)
I hope this helps .
If you have any more questions , please let me know
If you have more questions , or wish to discuss Jasper's conditions I would be more then happy to help.