The 40x objective is dirty

Professor Johnson takes a look at the slide and realizes that the previous class did not clean the objectives properly and you have oil on your 40x objective. He advices you to clean the objective thoroughly and try to re-focus your slide. 

As you are observing this wonderful ‘invisible life’ under the microscope you observe several small circular structures with a dark outline a bright center that you can’t identify. With excitement you ask you fellow students to have a look and help you identify it.