Above drawing shows microscope using immersion microscopy using only one slide, with instead of oil immersion, the lens of the microscope is immersed in the specimen solution.
Yes, the microscope lens is immersed not in oil, but in the solution itself with the specimen, the specimen you are attempting to view with the microscope. Read up on immersion microscopy to understand the differences and why these images are improved. Below is example of red blood cells I took using immersion photograph using a microscope. Note many more blurred objects can be seen, circled in yellow as the specimen depth of field and optical quality is a different versus having the air refraction.
One advantage is the the specimens are not compressed between two slides. This might be good for viewing live cells or live organisms.
A disadvantage, live organisms would move quite a bit and be difficult to track in the microscope.
Ideally you would have a solution like oil that has the same index of refraction of the glass slides. I assume the water solution has a different index of refraction than the glass.
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